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Subject[patch 48/50] md: fix prexor vs sync_request race
-stable review patch.  If anyone has any objections, please let us know.

From: Dan Williams <>

upstream commit: e0a115e5aa554b93150a8dc1c3fe15467708abb2

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: <>
Signed-off-by: Dan Williams <>
Signed-off-by: Neil Brown <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
[chrisw: backport to]
Signed-off-by: Chris Wright <>
drivers/md/raid5.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -2621,6 +2621,7 @@ static void handle_stripe5(struct stripe
struct stripe_head_state s;
struct r5dev *dev;
unsigned long pending = 0;
+ int prexor;

memset(&s, 0, sizeof(s));
pr_debug("handling stripe %llu, state=%#lx cnt=%d, pd_idx=%d "
@@ -2740,9 +2741,11 @@ static void handle_stripe5(struct stripe
/* 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);
@@ -2776,6 +2779,8 @@ static void handle_stripe5(struct stripe
if (!test_and_set_bit(
STRIPE_OP_IO, &sh->ops.pending))
+ if (prexor)
+ continue;
if (!test_bit(R5_Insync, &dev->flags) ||
(i == sh->pd_idx && s.failed == 0))
set_bit(STRIPE_INSYNC, &sh->state);

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