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md: fix prexor vs sync_request race 

During the initial array synchronization process there is a window between
when a prexor operation is scheduled to a specific stripe and when it
completes for a sync_request to be scheduled to the same stripe.  When
this happens the prexor completes and the stripe is unconditionally marked
"insync", effectively canceling the sync_request for the stripe.  Prior to
2.6.23 this was not a problem because the prexor operation was done under
sh->lock.  The effect in older kernels being that the prexor would still
erroneously mark the stripe "insync", but sync_request would be held off
and re-mark the stripe as "!in_sync".

Change the write completion logic to not mark the stripe "in_sync" if a
prexor was performed.  The effect of the change is to sometimes not set
STRIPE_INSYNC.  The worst this can do is cause the resync to stall waiting
for STRIPE_INSYNC to be set.  If this were happening, then STRIPE_SYNCING
would be set and handle_issuing_new_read_requests would cause all
available blocks to eventually be read, at which point prexor would never
be used on that stripe any more and STRIPE_INSYNC would eventually be set.

echo repair > /sys/block/mdN/md/sync_action will correct arrays that may
have lost this race.

Cc: <stable@kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Dan Williams 2008-06-05 22:45:52 -07:00 committed by Linus Torvalds
parent b2c8daddcb
commit e0a115e5aa
1 changed files with 5 additions and 0 deletions
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5
drivers/md/raid5.c
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@ -2645,6 +2645,7 @@ static void handle_stripe5(struct stripe_head *sh)
struct r5dev *dev;
unsigned long pending = 0;
mdk_rdev_t *blocked_rdev = NULL;
int prexor;
memset(&s, 0, sizeof(s));
pr_debug("handling stripe %llu, state=%#lx cnt=%d, pd_idx=%d "
@ -2774,9 +2775,11 @@ static void handle_stripe5(struct stripe_head *sh)
/* leave prexor set until postxor is done, allows us to distinguish
* a rmw from a rcw during biodrain
*/
prexor = 0;
if (test_bit(STRIPE_OP_PREXOR, &sh->ops.complete) &&
test_bit(STRIPE_OP_POSTXOR, &sh->ops.complete)) {
prexor = 1;
clear_bit(STRIPE_OP_PREXOR, &sh->ops.complete);
clear_bit(STRIPE_OP_PREXOR, &sh->ops.ack);
clear_bit(STRIPE_OP_PREXOR, &sh->ops.pending);
@ -2810,6 +2813,8 @@ static void handle_stripe5(struct stripe_head *sh)
if (!test_and_set_bit(
STRIPE_OP_IO, &sh->ops.pending))
sh->ops.count++;
if (prexor)
continue;
if (!test_bit(R5_Insync, &dev->flags) ||
(i == sh->pd_idx && s.failed == 0))
set_bit(STRIPE_INSYNC, &sh->state);