Hello Driver List:
I am seeing some peculiar behavior when trying to fail accesses through
the block device link. I was hoping y'all can help me understand what is
going on. I've written a disk driver for a device that will enter a read
only mode during certain error conditions. I'm testing the disk in this
read only state and I see some writes returned successful which should
be failed. I see correct behavior when the same writes are issued to
the character device link (i.e. the writes return as failed). I have
written a simple program which reproduces the issue. The program simply
opens the device, and uses the read(2) and write(2) system calls to
perform a 4K access. Interestingly enough when the read is removed from
the program the write returns an error correctly! I have verified that
bioerror(9F) is being called in my driver and that bioerror is setting
the b_error field of the block IO structure on failed DMAs. Any
explanation would be greatly appreciated.
Here are my test results (note that /dev/rdsk/c9d0p0 always exhibits
correct behavior):
4K write:
% a.out /dev/rdsk/c9d0p0
write() returned -1 errno 6 <-- Correct
% a.out /dev/dsk/c9d0p0
write() returned -1 errno 6 <-- Correct
4K read followed by a 4K write:
% a.out /dev/rdsk/c9d0p0
read() returned 4096 errno 0
write() returned -1 errno 6 <-- Correct
% a.out /dev/dsk/c9d0p0
read() returned 4096 errno 0
write() returned 4096 errno 0 <-- Incorrect! This write should fail
4K write followed by a 4K read:
% a.out /dev/rdsk/c9d0p0
write() returned -1 errno 6
read() returned 4096 errno 0 <-- Correct
% a.out /dev/dsk/c9d0p0
write() returned -1 errno 6
read() returned -1 errno 6 <-- Incorrect! The read should return
successful
And here is my test program:
#define BUF_SIZE 4096
int main(int argc, char **argv)
{
int fd, st;
char *buf;
if (argc < 2)
{
printf("usage: %s <filename>\n", argv[0]);
return 1;
}
buf = malloc(BUF_SIZE);
if (!buf)
{
return 1;
}
memset(buf, 0, BUF_SIZE);
fd = open(argv[1], O_RDWR | O_LARGEFILE | O_SYNC);
if (fd < 0)
{
printf("Unable to open file %s (%d)\n", argv[1], errno);
free(buf);
return 1;
}
st = write(fd, buf, BUF_SIZE);
printf("write() returned %d errno %d\n", st, errno);
errno = 0;
st = read(fd, buf, BUF_SIZE);
printf("read() returned %d errno %d\n", st, errno);
close(fd);
free(buf);
return 0;
}
Thank you,
Josh Morris
[email protected]
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