[PATCH] md: Improve comments about locking situation in raid5 make_request

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 14 insertions, 1 deletions

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index bb16ac231a40..8eded08411f3 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -1768,6 +1768,14 @@ static int make_request(request_queue_t *q, struct bio * bi)
                 if (likely(conf->expand_progress == MaxSector))
                         disks = conf->raid_disks;
                 else {
+                        /* spinlock is needed as expand_progress may be
+                         * 64bit on a 32bit platform, and so it might be
+                         * possible to see a half-updated value
+                         * Ofcourse expand_progress could change after
+                         * the lock is dropped, so once we get a reference
+                         * to the stripe that we think it is, we will have
+                         * to check again.
+                         */
                         spin_lock_irq(&conf->device_lock);
                         disks = conf->raid_disks;
                         if (logical_sector >= conf->expand_progress)
@@ -1791,7 +1799,12 @@ static int make_request(request_queue_t *q, struct bio * bi)
                 if (sh) {
                         if (unlikely(conf->expand_progress != MaxSector)) {
                                 /* expansion might have moved on while waiting for a
-                                 * stripe, so we much do the range check again.
+                                 * stripe, so we must do the range check again.
+                                 * Expansion could still move past after this
+                                 * test, but as we are holding a reference to
+                                 * 'sh', we know that if that happens,
+                                 *  STRIPE_EXPANDING will get set and the expansion
+                                 * won't proceed until we finish with the stripe.
                                  */
                                 int must_retry = 0;
                                 spin_lock_irq(&conf->device_lock);