+Cc Peter,
Hi Peter,
This patch should be RFC, I haven't saw such issue in real world though, I still
worry that it may happen. Could you help to have a look at it.
This the question I'd like to ask here is that: in a preemptible kernel, if
thread A add itself into wait queue with TASK_UNINTERRUPTIBLE status through
prepare_to_wait, after that, thread B do the preemption, if there are no waker
for thread A, will thread A sleep forever?
Thanks,
On 2016/4/28 2:09, Jaegeuk Kim wrote:
> Hi Chao,
>
> On Wed, Apr 27, 2016 at 09:41:48PM +0800, Chao Yu wrote:
>> From: Chao Yu <yuch...@huawei.com>
>>
>> The following condition can happen in a preemptible kernel, it may cause
>> checkpointer hunging.
>>
>> CPU0: CPU1:
>> - write_checkpoint
>> - do_checkpoint
>> - wait_on_all_pages_writeback
>> - f2fs_write_end_io
>> - wake_up
>> this is last writebacked page, but
>> no sleeper in sbi->cp_wait wait
>> queue, wake_up is not been called.
>> - prepare_to_wait(TASK_UNINTERRUPTIBLE)
>> Here, current task can been preempted,
>> but there will be no waker since last
>> write_end_io has bypassed wake_up. So
>> current task will sleep forever.
>> - io_schedule_timeout
>
> Well, io_schedule_timeout should work for this?
>
> Thanks,
>
>> Now we use spinlock to create a critical section to guarantee preemption
>> was disabled during racing in between wait_on_all_pages_writeback and
>> f2fs_write_end_io, so that above condition can be avoided.
>>
>> Signed-off-by: Chao Yu <yuch...@huawei.com>
>> ---
>> fs/f2fs/checkpoint.c | 14 +++++++++-----
>> fs/f2fs/data.c | 9 +++++++--
>> fs/f2fs/f2fs.h | 3 ++-
>> fs/f2fs/super.c | 1 +
>> 4 files changed, 19 insertions(+), 8 deletions(-)
>>
>> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
>> index bf040b5..817cda7 100644
>> --- a/fs/f2fs/checkpoint.c
>> +++ b/fs/f2fs/checkpoint.c
>> @@ -914,15 +914,19 @@ static void wait_on_all_pages_writeback(struct
>> f2fs_sb_info *sbi)
>> {
>> DEFINE_WAIT(wait);
>>
>> - for (;;) {
>> + spin_lock(&sbi->cp_wb_lock);
>> +
>> + while (get_pages(sbi, F2FS_WRITEBACK)) {
>> prepare_to_wait(&sbi->cp_wait, &wait, TASK_UNINTERRUPTIBLE);
>>
>> - if (!get_pages(sbi, F2FS_WRITEBACK))
>> - break;
>> + spin_unlock(&sbi->cp_wb_lock);
>> + io_schedule();
>> + spin_lock(&sbi->cp_wb_lock);
>>
>> - io_schedule_timeout(5*HZ);
>> + finish_wait(&sbi->cp_wait, &wait);
>> }
>> - finish_wait(&sbi->cp_wait, &wait);
>> +
>> + spin_unlock(&sbi->cp_wb_lock);
>> }
>>
>> static int do_checkpoint(struct f2fs_sb_info *sbi, struct cp_control *cpc)
>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>> index 38ce5d6..8faeada 100644
>> --- a/fs/f2fs/data.c
>> +++ b/fs/f2fs/data.c
>> @@ -59,6 +59,7 @@ static void f2fs_write_end_io(struct bio *bio)
>> {
>> struct f2fs_sb_info *sbi = bio->bi_private;
>> struct bio_vec *bvec;
>> + unsigned long flags;
>> int i;
>>
>> bio_for_each_segment_all(bvec, bio, i) {
>> @@ -74,8 +75,12 @@ static void f2fs_write_end_io(struct bio *bio)
>> dec_page_count(sbi, F2FS_WRITEBACK);
>> }
>>
>> - if (!get_pages(sbi, F2FS_WRITEBACK) && wq_has_sleeper(&sbi->cp_wait))
>> - wake_up(&sbi->cp_wait);
>> + if (!get_pages(sbi, F2FS_WRITEBACK)) {
>> + spin_lock_irqsave(&sbi->cp_wb_lock, flags);
>> + if (wq_has_sleeper(&sbi->cp_wait))
>> + wake_up(&sbi->cp_wait);
>> + spin_unlock_irqrestore(&sbi->cp_wb_lock, flags);
>> + }
>>
>> bio_put(bio);
>> }
>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>> index 0786a45..cf646b3 100644
>> --- a/fs/f2fs/f2fs.h
>> +++ b/fs/f2fs/f2fs.h
>> @@ -725,7 +725,8 @@ struct f2fs_sb_info {
>> struct rw_semaphore cp_rwsem; /* blocking FS operations */
>> struct rw_semaphore node_write; /* locking node writes */
>> struct mutex writepages; /* mutex for writepages() */
>> - wait_queue_head_t cp_wait;
>> + wait_queue_head_t cp_wait; /* for wait pages writeback */
>> + spinlock_t cp_wb_lock; /* for protect cp_wait */
>> unsigned long last_time[MAX_TIME]; /* to store time in jiffies */
>> long interval_time[MAX_TIME]; /* to store thresholds */
>>
>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>> index 19a85cf..8b25ac1 100644
>> --- a/fs/f2fs/super.c
>> +++ b/fs/f2fs/super.c
>> @@ -1436,6 +1436,7 @@ try_onemore:
>>
>> init_rwsem(&sbi->cp_rwsem);
>> init_waitqueue_head(&sbi->cp_wait);
>> + spin_lock_init(&sbi->cp_wb_lock);
>> init_sb_info(sbi);
>>
>> /* get an inode for meta space */
>> --
>> 2.7.2
>
> ------------------------------------------------------------------------------
> Find and fix application performance issues faster with Applications Manager
> Applications Manager provides deep performance insights into multiple tiers of
> your business applications. It resolves application problems quickly and
> reduces your MTTR. Get your free trial!
> https://ad.doubleclick.net/ddm/clk/302982198;130105516;z
> _______________________________________________
> Linux-f2fs-devel mailing list
> linux-f2fs-de...@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
> .
>
