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md/raid1: delay reads that could overtake behind-writes.

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When a raid1 array is configured to support write-behind
on some devices, it normally only reads from other devices.
If all devices are write-behind (because the rest have failed)
it is possible for a read request to be serviced before a
behind-write request, which would appear as data corruption.

So when forced to read from a WriteMostly device, wait for any
write-behind to complete, and don't start any more behind-writes.

Signed-off-by: NeilBrown <neilb@suse.de>
This commit is contained in:
NeilBrown 2010-03-31 11:21:44 +11:00
parent d754c5ae1f
commit e555190d82
3 changed files with 22 additions and 8 deletions
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4
drivers/md/bitmap.c
View file

@ -1356,7 +1356,8 @@ void bitmap_endwrite(struct bitmap *bitmap, sector_t offset, unsigned long secto
{
if (!bitmap) return;
if (behind) {
atomic_dec(&bitmap->behind_writes);
if (atomic_dec_and_test(&bitmap->behind_writes))
wake_up(&bitmap->behind_wait);
PRINTK(KERN_DEBUG "dec write-behind count %d/%d\n",
atomic_read(&bitmap->behind_writes), bitmap->max_write_behind);
}
@ -1680,6 +1681,7 @@ int bitmap_create(mddev_t *mddev)
atomic_set(&bitmap->pending_writes, 0);
init_waitqueue_head(&bitmap->write_wait);
init_waitqueue_head(&bitmap->overflow_wait);
init_waitqueue_head(&bitmap->behind_wait);
bitmap->mddev = mddev;

1
drivers/md/bitmap.h
View file

@ -240,6 +240,7 @@ struct bitmap {
atomic_t pending_writes; /* pending writes to the bitmap file */
wait_queue_head_t write_wait;
wait_queue_head_t overflow_wait;
wait_queue_head_t behind_wait;
struct sysfs_dirent *sysfs_can_clear;
};

25
drivers/md/raid1.c
View file

@ -857,6 +857,15 @@ static int make_request(mddev_t *mddev, struct bio * bio)
}
mirror = conf->mirrors + rdisk;
if (test_bit(WriteMostly, &mirror->rdev->flags) &&
bitmap) {
/* Reading from a write-mostly device must
* take care not to over-take any writes
* that are 'behind'
*/
wait_event(bitmap->behind_wait,
atomic_read(&bitmap->behind_writes) == 0);
}
r1_bio->read_disk = rdisk;
read_bio = bio_clone(bio, GFP_NOIO);
@ -934,10 +943,14 @@ static int make_request(mddev_t *mddev, struct bio * bio)
set_bit(R1BIO_Degraded, &r1_bio->state);
}
/* do behind I/O ? */
/* do behind I/O ?
* Not if there are too many, or cannot allocate memory,
* or a reader on WriteMostly is waiting for behind writes
* to flush */
if (bitmap &&
(atomic_read(&bitmap->behind_writes)
< mddev->bitmap_info.max_write_behind) &&
!waitqueue_active(&bitmap->behind_wait) &&
(behind_pages = alloc_behind_pages(bio)) != NULL)
set_bit(R1BIO_BehindIO, &r1_bio->state);
@ -2144,15 +2157,13 @@ static int stop(mddev_t *mddev)
{
conf_t *conf = mddev->private;
struct bitmap *bitmap = mddev->bitmap;
int behind_wait = 0;
/* wait for behind writes to complete */
while (bitmap && atomic_read(&bitmap->behind_writes) > 0) {
behind_wait++;
printk(KERN_INFO "raid1: behind writes in progress on device %s, waiting to stop (%d)\n", mdname(mddev), behind_wait);
set_current_state(TASK_UNINTERRUPTIBLE);
schedule_timeout(HZ); /* wait a second */
if (bitmap && atomic_read(&bitmap->behind_writes) > 0) {
printk(KERN_INFO "raid1: behind writes in progress on device %s, waiting to stop.\n", mdname(mddev));
/* need to kick something here to make sure I/O goes? */
wait_event(bitmap->behind_wait,
atomic_read(&bitmap->behind_writes) == 0);
}
raise_barrier(conf);