Questions about data protection in the kernel
Full disclosure, I am a software engineer for Arcserve, responsible for maintaining our linux kernel driver component of our data protection software suites. I am here looking for help seeking answers to two specific questions. Not knowing who to ask or really how to communicate some of my questions, it seemed most appropriate to start my journey here with kernel newbies. Why does the kernel only provide disk snapshot capabilities via Device Mapper (and lvm)? I am aware of multiple companies that offer linux products that come with their own kernel modules to provide full featured snapshot capabilities. Some of these linux offerings are decades old and some are open source now. If they were to require device mapper to be setup previously, it would be difficult to insert themselves into many environments. All of the drivers seem to perform the same hook by replacing the fops->submit_bio (previously known as q->make_request_fn, and more on this in the next question). It seems to me there is at least some demand for data protection functionality outside of device mapper, and this hook could be so much cleaner if it was officially supported by the kernel. (I would be thrilled to learn that the answer is "because nobody has volunteered to write it.") In context of Multi-Queue Block IO (blk-mq), what is the future of the older Single Queue interface? I have puzzled together some of blk-mq's history simply by interacting with it as it has become necessary; reading code and git commit messages. What I have gleaned so far has me wondering if and when the future will be Multi-Queue only. I have read as many LWN articles I can find on the subject, but the future of IO queuing is still unclear to me. Thank you for your time and helping me to learn how to engage with the linux kernel community. Any feedback on how and where to ask questions would also be greatly appreciated. _ [cid:arcserve-email-logo_566d469b-c8dc-46eb-909b-300e3f3e47a1.jpg]<https://arcserve.com/> Kai Meyer | Sr. Software Engineer Office: 801.871.2765 | Mobile: | kai.me...@arcserve.com arcserve.com<https://www.arcserve.com/> | Twitter<https://twitter.com/Arcserve> | LinkedIn<https://www.linkedin.com/company/arcserve/> | YouTube<https://www.youtube.com/user/arcserve> _ If you are not the intended recipient of this message or received it erroneously, please notify the sender and delete it, together with any attachments, and be advised that any dissemination or copying of this message is prohibited. ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org https://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: What does inconsistent lock state mean?
On 12/08/2011 07:47 AM, Srivatsa Bhat wrote:
2 things:
1. Documentation/lockdep-design.txt explains the cryptic lock state
symbols.
2. Please post the lockdep splat _exactly_ as it appears, and _in full_
(and without line-wrapping, if possible). Usually lockdep is
intelligent
enough to tell you the possible scenario that would lock up your
system.
That gives a very good clue, in case you find it difficult to make
out what
is wrong from the cryptic symbols.
Regards,
Srivatsa S. Bhat
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Oh, sorry. I suppose I only included things that made any sense to me.
If I were to hazard a guess after reading through the design doc, it
sounds like there's a problem with the context in which locks are being
acquired? That seems odd to me, since I don't get the inconsistent lock
state until I'm trying to spin_unlock
sblsnap_snapshot_table[i].sblsnap_lock (which is why I assume it's
listed as one that's currently held.
Dec 7 15:52:20 dev2 kernel: =
Dec 7 15:52:20 dev2 kernel: [ INFO: inconsistent lock state ]
Dec 7 15:52:20 dev2 kernel: 2.6.32-220.el6.x86_64.debug #1
Dec 7 15:52:20 dev2 kernel: -
Dec 7 15:52:20 dev2 kernel: inconsistent {IN-HARDIRQ-W} -
{HARDIRQ-ON-W} usage.
Dec 7 15:52:20 dev2 kernel: tee/1966 [HC0[0]:SC0[0]:HE1:SE1] takes:
Dec 7 15:52:20 dev2 kernel: (vblk-lock){?.-...}, at:
[a04c4d2a] sblsnap_snap_now+0x25a/0x2a0 [sblsnap]
Dec 7 15:52:20 dev2 kernel: {IN-HARDIRQ-W} state was registered at:
Dec 7 15:52:20 dev2 kernel: [810afbca]
__lock_acquire+0x77a/0x1570
Dec 7 15:52:20 dev2 kernel: [810b0a64] lock_acquire+0xa4/0x120
Dec 7 15:52:20 dev2 kernel: [81520c75]
_spin_lock_irqsave+0x55/0xa0
Dec 7 15:52:20 dev2 kernel: [a006b19b] blk_done+0x2b/0x110
[virtio_blk]
Dec 7 15:52:20 dev2 kernel: [a00401dc]
vring_interrupt+0x3c/0xd0 [virtio_ring]
Dec 7 15:52:20 dev2 kernel: [810ec080]
handle_IRQ_event+0x50/0x160
Dec 7 15:52:20 dev2 kernel: [810ee840]
handle_edge_irq+0xe0/0x170
Dec 7 15:52:20 dev2 kernel: [8100e059] handle_irq+0x49/0xa0
Dec 7 15:52:20 dev2 kernel: [81526cdc] do_IRQ+0x6c/0xf0
Dec 7 15:52:20 dev2 kernel: [8100ba93] ret_from_intr+0x0/0x16
Dec 7 15:52:20 dev2 kernel: [810148e2] default_idle+0x52/0xc0
Dec 7 15:52:20 dev2 kernel: [81009e0b] cpu_idle+0xbb/0x110
Dec 7 15:52:20 dev2 kernel: [81516623]
start_secondary+0x211/0x254
Dec 7 15:52:20 dev2 kernel: irq event stamp: 4699
Dec 7 15:52:20 dev2 kernel: hardirqs last enabled at (4699):
[81179e81] __kmalloc+0x241/0x330
Dec 7 15:52:20 dev2 kernel: hardirqs last disabled at (4698):
[81179d60] __kmalloc+0x120/0x330
Dec 7 15:52:20 dev2 kernel: softirqs last enabled at (4696):
[81076baa] __do_softirq+0x14a/0x200
Dec 7 15:52:20 dev2 kernel: softirqs last disabled at (4681):
[8100c30c] call_softirq+0x1c/0x30
Dec 7 15:52:20 dev2 kernel:
Dec 7 15:52:20 dev2 kernel: other info that might help us debug this:
Dec 7 15:52:20 dev2 kernel: 1 lock held by tee/1966:
Dec 7 15:52:20 dev2 kernel: #0:
(sblsnap_snapshot_table[i].sblsnap_lock){+.+.+.}, at:
[a04c4b7c] sblsnap_snap_now+0xac/0x2a0 [sblsnap]
Dec 7 15:52:20 dev2 kernel:
Dec 7 15:52:20 dev2 kernel: stack backtrace:
Dec 7 15:52:20 dev2 kernel: Pid: 1966, comm: tee Not tainted
2.6.32-220.el6.x86_64.debug #1
Dec 7 15:52:20 dev2 kernel: Call Trace:
Dec 7 15:52:20 dev2 kernel: [810ad947] ?
print_usage_bug+0x177/0x180
Dec 7 15:52:20 dev2 kernel: [810ae8ed] ? mark_lock+0x35d/0x430
Dec 7 15:52:20 dev2 kernel: [810afa59] ?
__lock_acquire+0x609/0x1570
Dec 7 15:52:20 dev2 kernel: [810ab31d] ?
trace_hardirqs_off+0xd/0x10
Dec 7 15:52:20 dev2 kernel: [815208e7] ?
_spin_unlock_irqrestore+0x67/0x80
Dec 7 15:52:20 dev2 kernel: [8106ec43] ?
release_console_sem+0x203/0x250
Dec 7 15:52:20 dev2 kernel: [810b0a64] ? lock_acquire+0xa4/0x120
Dec 7 15:52:20 dev2 kernel: [a04c4d2a] ?
sblsnap_snap_now+0x25a/0x2a0 [sblsnap]
Dec 7 15:52:20 dev2 kernel: [81520af6] ? _spin_lock+0x36/0x70
Dec 7 15:52:20 dev2 kernel: [a04c4d2a] ?
sblsnap_snap_now+0x25a/0x2a0 [sblsnap]
Dec 7 15:52:20 dev2 kernel: [a04c4d2a] ?
sblsnap_snap_now+0x25a/0x2a0 [sblsnap]
Dec 7 15:52:20 dev2 kernel: [a04c4de4] ?
sblsnap_patch_blkdev+0x74/0x120 [sblsnap]
Dec 7 15:52:20 dev2 kernel: [a04c4eaf] ?
sblsnap_get_snapshot+0x1f/0x60 [sblsnap]
Dec 7 15:52:20 dev2 kernel: [a04c4f59] ?
sblsnap_create_snapshot+0x69/0x120 [sblsnap]
Dec 7 15:52:20 dev2 kernel: [a04c53cb] ?
sblsnap_config_write+0x26b/0x2c0 [sblsnap]
Dec 7 15:52:20 dev2 kernel:
Re: VFAT i_pos value
On 12/02/2011 11:23 PM, OGAWA Hirofumi wrote:
OGAWA Hirofumihirof...@mail.parknet.co.jp writes:
Kai Meyerk...@gnukai.com writes:
Thanks for the helpful response. I'm not entirely sure I understand the
next part though. I hacked a dirty entry dumper tool:
#includestdio.h
#includelinux/msdos_fs.h
#includesys/types.h
#includesys/stat.h
#includefcntl.h
#includeunistd.h
#includestring.h
int main(int argc, char** argv)
{
off_t pos = atoi(argv[2]);
unsigned long block;
off_t sector;
unsigned int offset;
int fd = open(argv[1], O_RDONLY);
char buf[512];
struct msdos_dir_entry dirent;
block = pos / (4096 / 32);
sector = block * 8;
offset = pos % (4096 / 32);
printf(block %lu, sector %lu, offset %u\n, block, sector,
offset);
lseek(fd, sector * 512, SEEK_SET);
if (read(fd, buf, 512) 0) {
fprintf(stderr, Unable to read from device %s\n,
argv[1]);
return -1;
}
memcpy(dirent, buf + offset, sizeof(dirent));
printf(name %s\n, dirent.name);
printf(attr %u\n, dirent.attr);
printf(lcase %u\n, dirent.lcase);
printf(ctime_cs %u\n, dirent.ctime_cs);
printf(ctime %u\n, dirent.ctime);
printf(cdate %u\n, dirent.cdate);
printf(adate %u\n, dirent.adate);
printf(starthi %u\n, dirent.starthi);
printf(time %u\n, dirent.time);
printf(date %u\n, dirent.date);
printf(start %u\n, dirent.start);
printf(size %u\n, dirent.size);
}
Here's what it outputs:
./vfat_entry /dev/sblsnap0 523793
block 4092, sector 32736, offset 17
name
attr 255
lcase 255
ctime_cs 255
ctime 12799
cdate 12670
adate 8224
starthi 8224
time 23072
date 21061
start 32
size 2171155456
So, I take starthi, and shift 16 bits left, then and in the start value.
That should give me the byte address of the first cluster of the file,
correct?
Then I need to follow the cluster chain until I get a bad value.
It looks like wrong as dirent. Did you use 523793 really? If so, I think
523791 is correct value. :)
And I didn't mention about offset correctly. offset means number of
entries, not bytes offset. So, bytes offset is buf + offset * 32.
(32 == sizeof(struct msdos_dir_entry))
Thanks.
Ok, I fixed the buf + offset * 32. I have a new volume, so the error is now:
fat_get_cluster: invalid cluster chain (i_pos 523781)
I added a few lines at the end to print the start value:
pos = dirent.starthi 16;
pos |= dirent.start;
printf(next pos: %u\n, sector);
[root@dev1 sblsnap]# ./vfat_entry /dev/sblsnap0 523781
block 4092, sector 32736, offset 5
name 3~1 ZER
attr 32
lcase 0
ctime_cs 100
ctime 29092
cdate 16264
adate 16264
starthi 4
time 29092
date 16264
start 7427
size 37748736
next pos: 32736
[root@dev1 sblsnap]# ./vfat_entry /dev/sblsnap0 32736
block 255, sector 2040, offset 96
name
attr 0
lcase 0
ctime_cs 0
ctime 0
cdate 0
adate 0
starthi 0
time 0
date 0
start 0
size 0
next pos: 2040
Does that look like what would be causing my error? meaning, sector 2040
has bad data?
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What does inconsistent lock state mean?
I'm getting this when I try to spin_unlock a recently acquired lock with
spin_lock. IRQs are still somewhat of a mystery to me, and cryptic lock
state symbols (IN-HARDIRQ-W, HARDIRQ-ON-W) are unintelligible to me.
Dec 7 15:52:20 dev2 kernel: =
Dec 7 15:52:20 dev2 kernel: [ INFO: inconsistent lock state ]
Dec 7 15:52:20 dev2 kernel: 2.6.32-220.el6.x86_64.debug #1
Dec 7 15:52:20 dev2 kernel: -
Dec 7 15:52:20 dev2 kernel: inconsistent {IN-HARDIRQ-W} -
{HARDIRQ-ON-W} usage.
Looking at lockdep.c isn't giving me any help either. It's obfuscated
beyond my ability to grok by simply reading the code.
It seems like this portion should help me, but it doesn't
Dec 7 15:52:20 dev2 kernel: {IN-HARDIRQ-W} state was registered at:
Dec 7 15:52:20 dev2 kernel: [810afbca]
__lock_acquire+0x77a/0x1570
Dec 7 15:52:20 dev2 kernel: [810b0a64] lock_acquire+0xa4/0x120
Dec 7 15:52:20 dev2 kernel: [81520c75]
_spin_lock_irqsave+0x55/0xa0
Dec 7 15:52:20 dev2 kernel: [a006b19b] blk_done+0x2b/0x110
[virtio_blk]
Dec 7 15:52:20 dev2 kernel: [a00401dc]
vring_interrupt+0x3c/0xd0 [virtio_ring]
Dec 7 15:52:20 dev2 kernel: [810ec080]
handle_IRQ_event+0x50/0x160
Dec 7 15:52:20 dev2 kernel: [810ee840]
handle_edge_irq+0xe0/0x170
Dec 7 15:52:20 dev2 kernel: [8100e059] handle_irq+0x49/0xa0
Dec 7 15:52:20 dev2 kernel: [81526cdc] do_IRQ+0x6c/0xf0
Dec 7 15:52:20 dev2 kernel: [8100ba93] ret_from_intr+0x0/0x16
Dec 7 15:52:20 dev2 kernel: [810148e2] default_idle+0x52/0xc0
Dec 7 15:52:20 dev2 kernel: [81009e0b] cpu_idle+0xbb/0x110
Dec 7 15:52:20 dev2 kernel: [81516623]
start_secondary+0x211/0x254
Then later it tells me that I'm holding 1 lock, which is the one that I
mentioned at the beginning that was just recently locked.
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Re: VFAT i_pos value
On 12/01/2011 07:38 AM, OGAWA Hirofumi wrote: Kai Meyerk...@gnukai.com writes: I'm getting this error: FAT: Filesystem error (dev sblsnap0) fat_get_cluster: invalid cluster chain (i_pos 523791) I'm wondering if there was a way to figure out what sector is causing the error? I would like to try and track down what is changing that sector and fix the problem. Is there a straight forward way to convert i_pos to a sector value? I've been staring at the fat.c and fat.h code all morning, and I'm having trouble grok'ing the flow. The i_pos means directory entry (contains inode information in unix-fs) position, block number == i_pos / (logical-blocksize / 32) offset == i_pos (logical-blocksize / 32) the above position's directory entry contains information for problematic file. This is how to use i_pos information. FWIW, in this error case, the cluster chain in FAT table which is pointed by that entry, it has invalid cluster value. Thanks. If you would verify my math for me, I would appreciate it. In this case, my logical block size is 4096, because byte 13 of the 8Gb file system is 8, and I take that to be 8 * 512, which is 4096. So: block_number = 523791 / (4096 / 32) = 4092 offset = 523791 % (4096 / 32) = 15 // I assume you meant modulo in your original post, and not binary AND. So if the block_number is 4092, I would multiply that by 8 (sectors per logical block) to get the sector number: 32736 Does the error indicate that sector contains the corrupted data? Or is it the sector that contains the information that points to the corrupted data? Or is it something entirely different? -Kai Meyer ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Understanding kmap/kunmap
I want to be able to copy data into a struct bio *, so I use
bio_for_each_segment to loop through each bvec, like so:
void some_function(struct bio *bio, char *some_data) {
struct bio_vec *bvec;
int i;
unsigned int bio_so_far = 0;
bio_for_each_segment(bvec, bio, i) {
char *bio_buffer = __bio_kmap_atomic(bio, i, KM_USER0);
memcpy(bio_buffer, some_data + bio_so_far, bvec-bv_len);
__bio_kunmap_atomic(bio, KM_USER0);
bio_so_far += bvec-bv_len;
}
}
There's lots more to the function, but this is basically the distilled
version with out any extra stuff.
What I'm finding is that when the bio has multiple bvecs that share the
same page, only the first bvec's data actually gets copied back up to
user-space, the rest is garbage or null (meaning, what was there
already). For instance, I see a lot of bios from vfat that are 4096
bytes long but are comprised of 8 bvecs that are 512 bytes long that all
have an offset to the same page.
I've tried doing just one kmap_atomic on the page by keeping track of
what the last page I kmap'ed was, but that didn't fix the problem either.
Any documentation or high level explanation of kmap/kunmap or other
ideas to try are welcome.
-Kai Meyer
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Re: Understanding kmap/kunmap
Correction. The problem occurs when 8 bios of size 512 with 1 bvec each
all share the same page. I made a bad assumption previously.
-Kai Meyer
On 12/01/2011 10:49 AM, Kai Meyer wrote:
I want to be able to copy data into a struct bio *, so I use
bio_for_each_segment to loop through each bvec, like so:
void some_function(struct bio *bio, char *some_data) {
struct bio_vec *bvec;
int i;
unsigned int bio_so_far = 0;
bio_for_each_segment(bvec, bio, i) {
char *bio_buffer = __bio_kmap_atomic(bio, i, KM_USER0);
memcpy(bio_buffer, some_data + bio_so_far, bvec-bv_len);
__bio_kunmap_atomic(bio, KM_USER0);
bio_so_far += bvec-bv_len;
}
}
There's lots more to the function, but this is basically the distilled
version with out any extra stuff.
What I'm finding is that when the bio has multiple bvecs that share the
same page, only the first bvec's data actually gets copied back up to
user-space, the rest is garbage or null (meaning, what was there
already). For instance, I see a lot of bios from vfat that are 4096
bytes long but are comprised of 8 bvecs that are 512 bytes long that all
have an offset to the same page.
I've tried doing just one kmap_atomic on the page by keeping track of
what the last page I kmap'ed was, but that didn't fix the problem either.
Any documentation or high level explanation of kmap/kunmap or other
ideas to try are welcome.
-Kai Meyer
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Re: VFAT i_pos value
On 12/01/2011 12:20 PM, OGAWA Hirofumi wrote:
Kai Meyerk...@gnukai.com writes:
The i_pos means directory entry (contains inode information in unix-fs)
position,
block number == i_pos / (logical-blocksize / 32)
offset == i_pos (logical-blocksize / 32)
the above position's directory entry contains information for
problematic file. This is how to use i_pos information.
FWIW, in this error case, the cluster chain in FAT table which is
pointed by that entry, it has invalid cluster value.
Thanks.
If you would verify my math for me, I would appreciate it.
In this case, my logical block size is 4096, because byte 13 of the 8Gb
file system is 8, and I take that to be 8 * 512, which is 4096. So:
block_number = 523791 / (4096 / 32) = 4092
offset = 523791 % (4096 / 32) = 15 // I assume you meant modulo in your
original post, and not binary AND.
Whoops, you are right. (I forgot -1)
So if the block_number is 4092, I would multiply that by 8 (sectors per
logical block) to get the sector number:
32736
Right.
Does the error indicate that sector contains the corrupted data?
No.
Or is it the sector that contains the information that points to the
corrupted data?
Right.
The i_pos is pointing a directory entry (include/linux/msdos_fs.h:
struct msdos_dir_entry).
And starthi (if FAT32) and start contain the pointer to next cluster
number. That message was outputted when walking in cluster chain.
If you want to see actual corrupted data, you can check the cluster
chain by pointing from that directory entry.
Thanks.
Thanks for the helpful response. I'm not entirely sure I understand the
next part though. I hacked a dirty entry dumper tool:
#include stdio.h
#include linux/msdos_fs.h
#include sys/types.h
#include sys/stat.h
#include fcntl.h
#include unistd.h
#include string.h
int main(int argc, char** argv)
{
off_t pos = atoi(argv[2]);
unsigned long block;
off_t sector;
unsigned int offset;
int fd = open(argv[1], O_RDONLY);
char buf[512];
struct msdos_dir_entry dirent;
block = pos / (4096 / 32);
sector = block * 8;
offset = pos % (4096 / 32);
printf(block %lu, sector %lu, offset %u\n, block, sector,
offset);
lseek(fd, sector * 512, SEEK_SET);
if (read(fd, buf, 512) 0) {
fprintf(stderr, Unable to read from device %s\n,
argv[1]);
return -1;
}
memcpy(dirent, buf + offset, sizeof(dirent));
printf(name %s\n, dirent.name);
printf(attr %u\n, dirent.attr);
printf(lcase %u\n, dirent.lcase);
printf(ctime_cs %u\n, dirent.ctime_cs);
printf(ctime %u\n, dirent.ctime);
printf(cdate %u\n, dirent.cdate);
printf(adate %u\n, dirent.adate);
printf(starthi %u\n, dirent.starthi);
printf(time %u\n, dirent.time);
printf(date %u\n, dirent.date);
printf(start %u\n, dirent.start);
printf(size %u\n, dirent.size);
}
Here's what it outputs:
./vfat_entry /dev/sblsnap0 523793
block 4092, sector 32736, offset 17
name
attr 255
lcase 255
ctime_cs 255
ctime 12799
cdate 12670
adate 8224
starthi 8224
time 23072
date 21061
start 32
size 2171155456
So, I take starthi, and shift 16 bits left, then and in the start value.
That should give me the byte address of the first cluster of the file,
correct?
Then I need to follow the cluster chain until I get a bad value.
Thanks
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VFAT i_pos value
I'm getting this error: FAT: Filesystem error (dev sblsnap0) fat_get_cluster: invalid cluster chain (i_pos 523791) I'm wondering if there was a way to figure out what sector is causing the error? I would like to try and track down what is changing that sector and fix the problem. Is there a straight forward way to convert i_pos to a sector value? I've been staring at the fat.c and fat.h code all morning, and I'm having trouble grok'ing the flow. -Kai Meyer ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Freeing work_struct memory
I've got a bug I'm having trouble identifying. It seems like it could be
related to my work_struct usage. I essentially have this:
struct my_worker {
work_struct work;
/* some other data */
};
void worker_fn(struct work_struct *work)
{
struct my_worker *worker = container_of(work, struct my_worker,
work);
/* ... do some stuff ... */
kfree(worker);
}
void worker_caller()
{
struct my_worker *worker = kmalloc(sizeof(*worker), GFP_KERNEL);
INIT_WORK(worker-work, worker_fn);
/* ... add some other stuff to *worker ... */
schedule_work(worker-work);
}
I frequently get a kernel panic with a specific test, but the stack
trace is rarely the same, which seems to indicate to me that I'm
corrupting data somewhere. So my question is:
Can I free the memory for struct my_worker *worker inside worker_fn?
Or does the work_queue stuff need to continue to use the struct
work_struct work member after the end of worker_fn?
-Kai Meyer
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Generic I/O
I'm finding it's really simple to write generic I/O functions for block devices (via a struct block_device) to mimic the posix read() and write() functions (I have to supply the position, since I don't have a fd to keep a position for me, but that's perfectly ok). I've got a little hack that allows me to run synchronously or asynchronously, relying on submit_bio() to create the threads for me. My caller function has an atomic_t value that I set equal to the number of bios I want to submit. Then I pass a pointer to that atomic_t around to each of the bios which decrement it in the endio function for that bio. Then the caller does this: while(atomic_read(numbios) 0) msleep(1); I'm finding the msleep(1) is a really really really long time, relatively. It seems to work ok if I just have an empty loop, but it also seems to me like I'm re-inventing a wheel here. Are there mechanisms that are better suited for waiting for tasks to complete? Or even for generic block I/O functions? -Kai Meyer ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: Spinlocks and interrupts
On 11/09/2011 08:38 PM, Dave Hylands wrote:
Hi Kai,
On Wed, Nov 9, 2011 at 3:12 PM, Kai Meyerk...@gnukai.com wrote:
Ok, I need mutual exclusion on a data structure regardless of interrupts
and core. It sounds like it can be done by using a spinlock and
disabling interrupts, but you mention that spinlocks are intended to
provide mutual exclsion between interrupt context and non-interrupt
context. Should I be using a semaphore (mutex) instead?
It depends. If the function is only called from thread context, then
you probably want to use a mutex. If there is a possibility that it
might be called from interrupt context, then you can't use a mutex.
Also, remember that spin-locks are no-ops on a single processor
machine, so as coded, you have no protection on a single-processor
machine if you're calling from thread context.
To make sure I understand you, it sounds like there's two contexts I
need to be concerned about, thread context and interrupt context. As far
as I can be sure, this code will only run in thread context. If you
could verify for me that a block device's make request function is only
reached in thread context, then that would make me doubly sure.
Perhaps I could explain my problem with some code:
struct my_struct *get_data(spinlock_t *mylock, int ALLOC_DATA)
{
struct my_struct *mydata = NULL;
spin_lock(mylock);
if (test_bit(index, mybitmap))
mydata = retrieve_data();
if (!mydata ALLOC_DATA) {
mydata = alloc_data();
set_bit(index, mybitmap);
}
spin_unlock(mylock);
return mydata;
}
I need to prevent retrieve_data from being called if the index bit is
set in mybitmap and alloc_data has not completed, so I use a bitmap to
indicate that alloc_data has completed. I also need to protect
alloc_data from being run multiple times, so I use the spin_lock to
ensure that test_bit (and possibly retrieve_data) is not run while
alloc_data is being run (because it runs while the bit is cleared).
If alloc_data might block, then you can't disable interrupts and you
definitely shouldn't be using spinlocks.
alloc_data will call kmalloc(size, GFP_KERNEL), which I think may block,
so disabling irqs is out.
Between thread context and kmalloc with GFP_KERNEL, it sounds like your
suggestion would be to use a mutex. Is that correct?
-Kai Meyer
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Re: Spinlocks and interrupts
Well, I changed my code to use a mutex instead of a spinlock, and now I get:
BUG: scheduling while atomic: swapper/0/0x1001
All I changed was the spinlock_t to a struct mutex, and call mutex_init,
mutex_lock, and mutex_unlock where I was previously calling the
spin_lock variations. I'm confused. What does mutex_lock do besides set
values in an atomic_t?
-Kai Meyer
On 11/10/2011 10:02 AM, Kai Meyer wrote:
On 11/09/2011 08:38 PM, Dave Hylands wrote:
Hi Kai,
On Wed, Nov 9, 2011 at 3:12 PM, Kai Meyerk...@gnukai.com wrote:
Ok, I need mutual exclusion on a data structure regardless of interrupts
and core. It sounds like it can be done by using a spinlock and
disabling interrupts, but you mention that spinlocks are intended to
provide mutual exclsion between interrupt context and non-interrupt
context. Should I be using a semaphore (mutex) instead?
It depends. If the function is only called from thread context, then
you probably want to use a mutex. If there is a possibility that it
might be called from interrupt context, then you can't use a mutex.
Also, remember that spin-locks are no-ops on a single processor
machine, so as coded, you have no protection on a single-processor
machine if you're calling from thread context.
To make sure I understand you, it sounds like there's two contexts I
need to be concerned about, thread context and interrupt context. As far
as I can be sure, this code will only run in thread context. If you
could verify for me that a block device's make request function is only
reached in thread context, then that would make me doubly sure.
Perhaps I could explain my problem with some code:
struct my_struct *get_data(spinlock_t *mylock, int ALLOC_DATA)
{
struct my_struct *mydata = NULL;
spin_lock(mylock);
if (test_bit(index, mybitmap))
mydata = retrieve_data();
if (!mydata ALLOC_DATA) {
mydata = alloc_data();
set_bit(index, mybitmap);
}
spin_unlock(mylock);
return mydata;
}
I need to prevent retrieve_data from being called if the index bit is
set in mybitmap and alloc_data has not completed, so I use a bitmap to
indicate that alloc_data has completed. I also need to protect
alloc_data from being run multiple times, so I use the spin_lock to
ensure that test_bit (and possibly retrieve_data) is not run while
alloc_data is being run (because it runs while the bit is cleared).
If alloc_data might block, then you can't disable interrupts and you
definitely shouldn't be using spinlocks.
alloc_data will call kmalloc(size, GFP_KERNEL), which I think may block,
so disabling irqs is out.
Between thread context and kmalloc with GFP_KERNEL, it sounds like your
suggestion would be to use a mutex. Is that correct?
-Kai Meyer
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Re: Spinlocks and interrupts
I think I get it. I'm hitting the scheduling while atomic because I'm
calling my function from a struct bio's endio function, which is
probably running with a lock held somewhere else, and then my mutex
sleeps, while the spin_lock functions do not sleep.
Perhaps I need to learn more about the context in which my endio
function is being called.
On 11/10/2011 11:02 AM, Kai Meyer wrote:
Well, I changed my code to use a mutex instead of a spinlock, and now I get:
BUG: scheduling while atomic: swapper/0/0x1001
All I changed was the spinlock_t to a struct mutex, and call mutex_init,
mutex_lock, and mutex_unlock where I was previously calling the
spin_lock variations. I'm confused. What does mutex_lock do besides set
values in an atomic_t?
-Kai Meyer
On 11/10/2011 10:02 AM, Kai Meyer wrote:
On 11/09/2011 08:38 PM, Dave Hylands wrote:
Hi Kai,
On Wed, Nov 9, 2011 at 3:12 PM, Kai Meyerk...@gnukai.comwrote:
Ok, I need mutual exclusion on a data structure regardless of interrupts
and core. It sounds like it can be done by using a spinlock and
disabling interrupts, but you mention that spinlocks are intended to
provide mutual exclsion between interrupt context and non-interrupt
context. Should I be using a semaphore (mutex) instead?
It depends. If the function is only called from thread context, then
you probably want to use a mutex. If there is a possibility that it
might be called from interrupt context, then you can't use a mutex.
Also, remember that spin-locks are no-ops on a single processor
machine, so as coded, you have no protection on a single-processor
machine if you're calling from thread context.
To make sure I understand you, it sounds like there's two contexts I
need to be concerned about, thread context and interrupt context. As far
as I can be sure, this code will only run in thread context. If you
could verify for me that a block device's make request function is only
reached in thread context, then that would make me doubly sure.
Perhaps I could explain my problem with some code:
struct my_struct *get_data(spinlock_t *mylock, int ALLOC_DATA)
{
struct my_struct *mydata = NULL;
spin_lock(mylock);
if (test_bit(index, mybitmap))
mydata = retrieve_data();
if (!mydataALLOC_DATA) {
mydata = alloc_data();
set_bit(index, mybitmap);
}
spin_unlock(mylock);
return mydata;
}
I need to prevent retrieve_data from being called if the index bit is
set in mybitmap and alloc_data has not completed, so I use a bitmap to
indicate that alloc_data has completed. I also need to protect
alloc_data from being run multiple times, so I use the spin_lock to
ensure that test_bit (and possibly retrieve_data) is not run while
alloc_data is being run (because it runs while the bit is cleared).
If alloc_data might block, then you can't disable interrupts and you
definitely shouldn't be using spinlocks.
alloc_data will call kmalloc(size, GFP_KERNEL), which I think may block,
so disabling irqs is out.
Between thread context and kmalloc with GFP_KERNEL, it sounds like your
suggestion would be to use a mutex. Is that correct?
-Kai Meyer
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Re: Spinlocks and interrupts
Alright, to summarize, for my benefit mostly, I'm writing a block device driver, which has 2 entry points into my code that will reach this critical section. It's either the make request function for the block device, or the resulting bio-bi_end_io function. I do some waiting with msleep() (for now) from the make request function entry point, so I'm confident that entry point is not in an atomic context. I also only end up requesting the critical section to call kmalloc from this context, which is why I never ran into the scheduling while atomic issue before. I'm fairly certain the critical section executes in thread context not interrupt context from either entry point. I'm certain that the spinlock_t is only ever used in one function (a I posted a simplified version of the critical section earlier). It seems that the critical section is often called in an atomic context. The spin_lock function sounds like it will only cause a second call to spin_lock to spin if it is called on a separate core. But, since I'm certain the critical section is never called from interrupt context, only thread context, the fact that pre-emption is disabled on the core should provide the protection I need with out having to disable IRQs. Disabling IRQs would prevent an interrupt from occurring while the lock is acquired. I would like to avoid disabling interrupts if I don't need to. So it sounds like spin_lock/spin_unlock is the correct choice? In addition, I'd like to be more confident in my assumptions above. Can I test for atomic context? For instance, I know that you can call irqs_disabled(), is there a similar is_atomic() function I can call? I would like to put a few calls in different places to learn what sort of context I'm. -Kai Meyer On 11/10/2011 12:19 PM, Jeff Haran wrote: -Original Message- From: kernelnewbies-bounces+jharan=bytemobile@kernelnewbies.org [mailto:kernelnewbies- bounces+jharan=bytemobile@kernelnewbies.org] On Behalf Of Dave Hylands Sent: Thursday, November 10, 2011 11:07 AM To: Kai Meyer Cc: kernelnewbies@kernelnewbies.org Subject: Re: Spinlocks and interrupts Hi Kai, On Thu, Nov 10, 2011 at 10:14 AM, Kai Meyerk...@gnukai.com wrote: I think I get it. I'm hitting the scheduling while atomic because I'm calling my function from a struct bio's endio function, which is probably running with a lock held somewhere else, and then my mutex sleeps, while the spin_lock functions do not sleep. Actually, just holding a lock doesn't create an atomic context. I believe on kernels with kernel pre-emption enabled the act of taking the lock disables pre-emption. If it didn't work this way you could end up taking the lock in one process context and while the lock was held get pre-empted. Then another process tries to take the lock and you dead lock. Jeff Haran ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: Spinlocks and interrupts
On 11/10/2011 04:00 PM, Jeff Haran wrote: -Original Message- From: kernelnewbies-boun...@kernelnewbies.org [mailto:kernelnewbies- boun...@kernelnewbies.org] On Behalf Of Kai Meyer Sent: Thursday, November 10, 2011 1:55 PM To: kernelnewbies@kernelnewbies.org Subject: Re: Spinlocks and interrupts Alright, to summarize, for my benefit mostly, I'm writing a block device driver, which has 2 entry points into my code that will reach this critical section. It's either the make request function for the block device, or the resulting bio-bi_end_io function. I do some waiting with msleep() (for now) from the make request function entry point, so I'm confident that entry point is not in an atomic context. I also only end up requesting the critical section to call kmalloc from this context, which is why I never ran into the scheduling while atomic issue before. I'm fairly certain the critical section executes in thread context not interrupt context from either entry point. I'm certain that the spinlock_t is only ever used in one function (a I posted a simplified version of the critical section earlier). It seems that the critical section is often called in an atomic context. The spin_lock function sounds like it will only cause a second call to spin_lock to spin if it is called on a separate core. But, since I'm certain the critical section is never called from interrupt context, only thread context, the fact that pre-emption is disabled on the core should provide the protection I need with out having to disable IRQs. Disabling IRQs would prevent an interrupt from occurring while the lock is acquired. I would like to avoid disabling interrupts if I don't need to. So it sounds like spin_lock/spin_unlock is the correct choice? In addition, I'd like to be more confident in my assumptions above. Can I test for atomic context? For instance, I know that you can call irqs_disabled(), is there a similar is_atomic() function I can call? I would like to put a few calls in different places to learn what sort of context I'm. -Kai Meyer On 11/10/2011 12:19 PM, Jeff Haran wrote: -Original Message- From: kernelnewbies- bounces+jharan=bytemobile@kernelnewbies.org [mailto:kernelnewbies- bounces+jharan=bytemobile@kernelnewbies.org] On Behalf Of Dave Hylands Sent: Thursday, November 10, 2011 11:07 AM To: Kai Meyer Cc: kernelnewbies@kernelnewbies.org Subject: Re: Spinlocks and interrupts Hi Kai, On Thu, Nov 10, 2011 at 10:14 AM, Kai Meyerk...@gnukai.com wrote: I think I get it. I'm hitting the scheduling while atomic because I'm calling my function from a struct bio's endio function, which is probably running with a lock held somewhere else, and then my mutex sleeps, while the spin_lock functions do not sleep. Actually, just holding a lock doesn't create an atomic context. I believe on kernels with kernel pre-emption enabled the act of taking the lock disables pre-emption. If it didn't work this way you could end up taking the lock in one process context and while the lock was held get pre-empted. Then another process tries to take the lock and you dead lock. Jeff Haran Kai, you might want to try bottom posting. It is the standard on these lists. It makes it easier for others to follow the thread. I know of no kernel call that you can make to test for current execution context. There are the in_irq(), in_interrupt() and in_softirq() macros in hardirq.h, but when I've looked at the code that implements them I've come to the conclusion that they sometimes will lie. in_softirq() returns non-zero if you are in a software IRQ. Fair enough. But based on my reading in the past it's looked to me like it will also return non-zero if you've disabled bottom halves from process context with say a call to spin_lock_bh(). It would be nice if there were some way of asking the kernel what context you are in, for debugging if for no other reason, but if it's there I haven't found it. I'd love to be proven wrong here, BTW. If others know better, please enlighten me. Jeff Haran ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies I try to remember to bottom post on message lists, but obviously I've been negligent :) Perhaps I'll just add some calls to msleep() at various places to help me identify when portions of my code are in an atomic context, just to help me learn what's going on. -Kai Meyer ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Spinlocks and interrupts
When I readup on spinlocks, it seems like I need to choose between disabling interrupts and not. If a spinlock_t is never used during an interrupt, am I safe to leave interrupts enabled while I hold the lock? (Same question for read/write locks if it is different.) -Kai Meyer ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: Spinlocks and interrupts
Ok, I need mutual exclusion on a data structure regardless of interrupts
and core. It sounds like it can be done by using a spinlock and
disabling interrupts, but you mention that spinlocks are intended to
provide mutual exclsion between interrupt context and non-interrupt
context. Should I be using a semaphore (mutex) instead?
Perhaps I could explain my problem with some code:
struct my_struct *get_data(spinlock_t *mylock, int ALLOC_DATA)
{
struct my_struct *mydata = NULL;
spin_lock(mylock);
if (test_bit(index, mybitmap))
mydata = retrieve_data();
if (!mydata ALLOC_DATA) {
mydata = alloc_data();
set_bit(index, mybitmap);
}
spin_unlock(mylock);
return mydata;
}
I need to prevent retrieve_data from being called if the index bit is
set in mybitmap and alloc_data has not completed, so I use a bitmap to
indicate that alloc_data has completed. I also need to protect
alloc_data from being run multiple times, so I use the spin_lock to
ensure that test_bit (and possibly retrieve_data) is not run while
alloc_data is being run (because it runs while the bit is cleared).
-Kai Meyer
On 11/09/2011 02:40 PM, Dave Hylands wrote:
Hi Kai,
On Wed, Nov 9, 2011 at 1:07 PM, Kai Meyerk...@gnukai.com wrote:
When I readup on spinlocks, it seems like I need to choose between
disabling interrupts and not. If a spinlock_t is never used during an
interrupt, am I safe to leave interrupts enabled while I hold the lock?
(Same question for read/write locks if it is different.)
So the intention behind using a spinlock is to provide mutual exclusion.
A spinlock by itself only really provides mutual exclusion between 2
cores, and not within the same core. To provide the mutual exclusion
within the same core, you need to disable interrupts.
Normally, you would disable interrupts and acquire the spinlock to
guarantee that mutual exclusion, and the only reason you would
normally use the spinlock without disabling interrupts is when you
know that interrupts are already disabled.
The danger of acquiring a spinlock with interrupts enabled is that if
another interrupt fired (or the same interrupt fired again) and it
tried to acquire the same spinlock, then you could have deadlock.
If no interrupts touch the spinlock, then you're probably using the
wrong mutual exclusion mechanism. spinlocks are really intended to
provide mutual exclsion between interrupt context and non-interrupt
context.
Also remember, that on a non-SMP (aka UP) build, spinlocks become
no-ops (except when certain debug checking code is enabled).
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Re: Understanding memcpy
Thanks for catching that :) I knew it would be something simple.
On 10/19/2011 10:00 PM, rohan puri wrote:
On Thu, Oct 20, 2011 at 3:04 AM, Kai Meyer k...@gnukai.com
mailto:k...@gnukai.com wrote:
I'm trying to poke around an ext4 file system. I can submit a bio for
the correct block, and read in what seems to be the correct
information,
but when I try to memcpy my char *buffer to a reference to a
struct I've
made, it just doesn't seem to work. The relevant code looks like this:
typedef struct ext2_superblock {
/* 00-03 */ uint32_t e2sb_inode_count;
/* 04-07 */ uint32_t e2sb_block_count;
/* 08-11 */ uint32_t e2sb_blocks_reserved;
/* 12-15 */ uint32_t e2sb_unallocated_blocks;
/* 16-19 */ uint32_t e2sb_unallocated_inodes;
/* 20-23 */ uint32_t e2sb_sb_block;
/* 24-27 */ uint32_t e2sb_log_block_size;
/* 28-31 */ uint32_t e2sb_log_fragment_size;
/* 32-35 */ uint32_t e2sb_num_blocks_per_group;
/* 36-39 */ uint32_t e2sb_num_frag_per_group;
/* 40-43 */ uint32_t e2sb_num_inodes_per_group;
/* 44-47 */ uint32_t e2sb_last_mount_time;
/* 48-51 */ uint32_t e2sb_last_written_time;
/* 52-53 */ uint16_t e2sb_num_mounted;
/* 54-55 */ uint16_t e2sb_num_allowed_mounts;
/* 56-57 */ uint16_t e2sb_signature;
/* 58-59 */ uint16_t e2sb_fs_state;
/* 60-61 */ uint16_t e2sb_error_action;
/* 62-63 */ uint16_t e2sb_ver_minor;
/* 64-67 */ uint32_t e2sb_last_check;
/* 68-71 */ uint32_t e2sb_time_between_checks;
/* 72-75 */ uint32_t e2sb_os_id;
/* 76-79 */ uint32_t e2sb_ver_major;
/* 80-81 */ uint16_t e2sb_uid;
/* 82-83 */ uint16_t e2sb_gid;
} e2sb;
char *buffer;
uint32_t *pointer;
e2sb sb;
buffer = __bio_kmap_atomic(bio, 0, KM_USER0);
pointer = (uint32_t *)buffer;
printk(KERN_DEBUG sizeof pbd-sb %lu\n, sizeof(bpd-sb));
printk(KERN_DEBUG Inode Count: %u\n, pointer[0]); /* Works! */
printk(KERN_DEBUG Block Count: %u\n, pointer[1]); /* Works! */
printk(KERN_DEBUG Block Reserved: %u\n, pointer[2]); /* Works! */
printk(KERN_DEBUG Unallocated blocks: %u\n, pointer[3]); /*
Works! */
printk(KERN_DEBUG Unallocated inodes: %u\n, pointer[4]); /*
Works! */
memcpy(buffer, sb, sizeof(sb));
This should be : -
memcpy(sb, buffer, sizeof(sb));
__bio_kunmap_atomic(bio, KM_USER0);
printk(KERN_DEBUG e2sb_debug: Total number of inodes in file system
%u\n, sb-e2sb_inode_count);/* Doesn't work! */
printk(KERN_DEBUG e2sb_debug: Total number of blocks in file
system%u\n, sb-e2sb_block_count); /* Doesn't work! */
My code is actually much more verbose. The values I get from indexing
into pointer are correct, and match what I get from dumpe2fs. The
values
I get from the e2sb struct are not. They are usually 0. I would
imagine
that memcpy is the fastest way to copy data from buffer instead of
casting the pointer to something else, and using array indexing to get
the values.
I struggled to find where ext4 actually does this, so I'm making
this up
as I go along. Any thing that you see that I should be doing a
different
way that isn't actually part of my question is welcome too.
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Regards,
Rohan Puri
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Re: Trouble removing character device
I do call unregister_chrdev_region. There are 5 functions in my original email that I call during the life of my module. Still no luck. -Kai Meyer On 10/19/2011 09:47 PM, rohan puri wrote: On Thu, Oct 20, 2011 at 2:25 AM, Kai Meyer k...@gnukai.com mailto:k...@gnukai.com wrote: Unfortunately I can't share the source code, it belongs to the company I work for. All of cdev_init, cdev_del, and unregister_chrdev_region are void functions, so they have no return value. I check the return value of alloc_chrdev_region and cdev_add and check for errors with BUG_ON (for now). -Kai Meyer On 10/19/2011 02:18 PM, Daniel Baluta wrote: On Wed, Oct 19, 2011 at 7:04 PM, Kai Meyerk...@gnukai.com mailto:k...@gnukai.com wrote: I can't seem to get my character device to remove itself from the /proc/devices list. I'm calling all of the following functions like so: alloc_chrdev_region(dev, 0, 5, my_char); cdev_init(my_cdev,my_ops); cdev_add(my_cdev, MKDEV(my_major, my_minor), 1); cdev_del(my_cdev); unregister_chrdev_region(my_major, 5); It seems like I'm missing something, but I can't find it. I'm referencing the Linux Device Drivers v3, chapter 3. In the example code, the scull_cleanup_module function calls cdev_dell and unregister_chrdev_region, just like I do. To be clear, after I unload my module (after calling cdev_del and unregister_chrdev_region), my my_char string still shows up in /proc/devices. Did you check return codes for all functions? Also, can you post a link to the code? thanks, Daniel. ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org mailto:Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org mailto:Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies During cleanup i think you need to call function unregister_chrdev_region(). Regards, Rohan Puri ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Trouble removing character device
I can't seem to get my character device to remove itself from the /proc/devices list. I'm calling all of the following functions like so: alloc_chrdev_region(dev, 0, 5, my_char); cdev_init(my_cdev, my_ops); cdev_add(my_cdev, MKDEV(my_major, my_minor), 1); cdev_del(my_cdev); unregister_chrdev_region(my_major, 5); It seems like I'm missing something, but I can't find it. I'm referencing the Linux Device Drivers v3, chapter 3. In the example code, the scull_cleanup_module function calls cdev_dell and unregister_chrdev_region, just like I do. To be clear, after I unload my module (after calling cdev_del and unregister_chrdev_region), my my_char string still shows up in /proc/devices. -Kai Meyer ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Understanding memcpy
I'm trying to poke around an ext4 file system. I can submit a bio for
the correct block, and read in what seems to be the correct information,
but when I try to memcpy my char *buffer to a reference to a struct I've
made, it just doesn't seem to work. The relevant code looks like this:
typedef struct ext2_superblock {
/* 00-03 */ uint32_t e2sb_inode_count;
/* 04-07 */ uint32_t e2sb_block_count;
/* 08-11 */ uint32_t e2sb_blocks_reserved;
/* 12-15 */ uint32_t e2sb_unallocated_blocks;
/* 16-19 */ uint32_t e2sb_unallocated_inodes;
/* 20-23 */ uint32_t e2sb_sb_block;
/* 24-27 */ uint32_t e2sb_log_block_size;
/* 28-31 */ uint32_t e2sb_log_fragment_size;
/* 32-35 */ uint32_t e2sb_num_blocks_per_group;
/* 36-39 */ uint32_t e2sb_num_frag_per_group;
/* 40-43 */ uint32_t e2sb_num_inodes_per_group;
/* 44-47 */ uint32_t e2sb_last_mount_time;
/* 48-51 */ uint32_t e2sb_last_written_time;
/* 52-53 */ uint16_t e2sb_num_mounted;
/* 54-55 */ uint16_t e2sb_num_allowed_mounts;
/* 56-57 */ uint16_t e2sb_signature;
/* 58-59 */ uint16_t e2sb_fs_state;
/* 60-61 */ uint16_t e2sb_error_action;
/* 62-63 */ uint16_t e2sb_ver_minor;
/* 64-67 */ uint32_t e2sb_last_check;
/* 68-71 */ uint32_t e2sb_time_between_checks;
/* 72-75 */ uint32_t e2sb_os_id;
/* 76-79 */ uint32_t e2sb_ver_major;
/* 80-81 */ uint16_t e2sb_uid;
/* 82-83 */ uint16_t e2sb_gid;
} e2sb;
char *buffer;
uint32_t *pointer;
e2sb sb;
buffer = __bio_kmap_atomic(bio, 0, KM_USER0);
pointer = (uint32_t *)buffer;
printk(KERN_DEBUG sizeof pbd-sb %lu\n, sizeof(bpd-sb));
printk(KERN_DEBUG Inode Count: %u\n, pointer[0]); /* Works! */
printk(KERN_DEBUG Block Count: %u\n, pointer[1]); /* Works! */
printk(KERN_DEBUG Block Reserved: %u\n, pointer[2]); /* Works! */
printk(KERN_DEBUG Unallocated blocks: %u\n, pointer[3]); /* Works! */
printk(KERN_DEBUG Unallocated inodes: %u\n, pointer[4]); /* Works! */
memcpy(buffer, sb, sizeof(sb));
__bio_kunmap_atomic(bio, KM_USER0);
printk(KERN_DEBUG e2sb_debug: Total number of inodes in file system
%u\n, sb-e2sb_inode_count);/* Doesn't work! */
printk(KERN_DEBUG e2sb_debug: Total number of blocks in file
system%u\n, sb-e2sb_block_count); /* Doesn't work! */
My code is actually much more verbose. The values I get from indexing
into pointer are correct, and match what I get from dumpe2fs. The values
I get from the e2sb struct are not. They are usually 0. I would imagine
that memcpy is the fastest way to copy data from buffer instead of
casting the pointer to something else, and using array indexing to get
the values.
I struggled to find where ext4 actually does this, so I'm making this up
as I go along. Any thing that you see that I should be doing a different
way that isn't actually part of my question is welcome too.
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Re: how diff between hardlink trees works?
On 09/09/2011 09:05 AM, Vaibhav Jain wrote: Hi, I am not able to understand how diff between two trees of which one is just contains hardlinks to another's files (cp -al )ing works.I am asking this question here because I need to build a custom kernel for which I need to generate patch. So the documentation suggests to create a hardlink copy of the kernel source tree using cp -al and then make changes to one of the trees and run a diff.I am wondering that if files are hardlinks then changes to one copy will affect another in which case diff should give no output. Also, the patch I created looks a little odd as it contains complete modified files instead of just the differences. Please help! Thanks Vaibhav Jain ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies Make the hard link copy like normal. Then delete the directory that you are making changes to (in the hard link directory), then copy the files over with out hard links. That way most of the kernel tree is hard linked, and just the portion you want to work on is a copy. That way the diff will work. Otherwise, skip the hard link part all together, and just make a full copy. Uses lots of disk space and takes longer to diff. -Kai Meyer ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Filesystem allocation bitmap
Is there a generic filesystem method to retrieve the filesystem's allocation bitmap? I'm mostly interested in Ext filesystems, so if there's nothing generic, I'm happy with a specific solution for just Ext. If the answer is read https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout and then read the bitmaps directly from disk, I think I can deal with that too. -Kai Meyer ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
