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

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>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 49d6080387c8..c9c6a345e17b 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -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;