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Subject[110/111] md/raid1: delay reads that could overtake behind-writes.
2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------

From: NeilBrown <neilb@suse.de>

commit e555190d82c0f58e825e3cbd9e6ebe2e7ac713bd upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>


---
drivers/md/bitmap.c | 4 +++-
drivers/md/bitmap.h | 3 +++
drivers/md/raid1.c | 25 ++++++++++++++++++-------
3 files changed, 24 insertions(+), 8 deletions(-)

--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -1317,7 +1317,8 @@ void bitmap_endwrite(struct bitmap *bitm
{
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);
}
@@ -1629,6 +1630,7 @@ 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;

--- a/drivers/md/bitmap.h
+++ b/drivers/md/bitmap.h
@@ -254,6 +254,9 @@ struct bitmap {
wait_queue_head_t write_wait;
wait_queue_head_t overflow_wait;

+#ifndef __GENKSYMS__
+ wait_queue_head_t behind_wait;
+#endif
};

/* the bitmap API */
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -845,6 +845,15 @@ static int make_request(struct request_q
}
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);
@@ -922,9 +931,13 @@ static int make_request(struct request_q
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) < bitmap->max_write_behind &&
+ !waitqueue_active(&bitmap->behind_wait) &&
(behind_pages = alloc_behind_pages(bio)) != NULL)
set_bit(R1BIO_BehindIO, &r1_bio->state);

@@ -2105,15 +2118,13 @@ 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);



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