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authorLinus Torvalds <torvalds@linux-foundation.org>2016-04-27 12:03:59 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2016-04-27 12:03:59 -0700
commitb75a2bf899b668b1d52de8846aafdbcf81349c73 (patch)
tree6696988bd5a31543833d4b275605d766e0a20840 /kernel/workqueue.c
parent763cfc86ee8fd728a7cf2334b8d3a897af7a7ade (diff)
parent346c09f80459a3ad97df1816d6d606169a51001a (diff)
downloadlinux-sh-b75a2bf899b668b1d52de8846aafdbcf81349c73.tar.gz
Merge branch 'for-4.6-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq
Pull workqueue fix from Tejun Heo: "So, it turns out we had a silly bug in the most fundamental part of workqueue for a very long time. AFAICS, this dates back to pre-git era and has quite likely been there from the time workqueue was first introduced. A work item uses its PENDING bit to synchronize multiple queuers. Anyone who wins the PENDING bit owns the pending state of the work item. Whether a queuer wins or loses the race, one thing should be guaranteed - there will soon be at least one execution of the work item - where "after" means that the execution instance would be able to see all the changes that the queuer has made prior to the queueing attempt. Unfortunately, we were missing a smp_mb() after clearing PENDING for execution, so nothing guaranteed visibility of the changes that a queueing loser has made, which manifested as a reproducible blk-mq stall. Lots of kudos to Roman for debugging the problem. The patch for -stable is the minimal one. For v3.7, Peter is working on a patch to make the code path slightly more efficient and less fragile" * 'for-4.6-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq: workqueue: fix ghost PENDING flag while doing MQ IO
Diffstat (limited to 'kernel/workqueue.c')
-rw-r--r--kernel/workqueue.c29
1 files changed, 29 insertions, 0 deletions
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 2232ae3e3ad6..3bfdff06eea7 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -666,6 +666,35 @@ static void set_work_pool_and_clear_pending(struct work_struct *work,
*/
smp_wmb();
set_work_data(work, (unsigned long)pool_id << WORK_OFFQ_POOL_SHIFT, 0);
+ /*
+ * The following mb guarantees that previous clear of a PENDING bit
+ * will not be reordered with any speculative LOADS or STORES from
+ * work->current_func, which is executed afterwards. This possible
+ * reordering can lead to a missed execution on attempt to qeueue
+ * the same @work. E.g. consider this case:
+ *
+ * CPU#0 CPU#1
+ * ---------------------------- --------------------------------
+ *
+ * 1 STORE event_indicated
+ * 2 queue_work_on() {
+ * 3 test_and_set_bit(PENDING)
+ * 4 } set_..._and_clear_pending() {
+ * 5 set_work_data() # clear bit
+ * 6 smp_mb()
+ * 7 work->current_func() {
+ * 8 LOAD event_indicated
+ * }
+ *
+ * Without an explicit full barrier speculative LOAD on line 8 can
+ * be executed before CPU#0 does STORE on line 1. If that happens,
+ * CPU#0 observes the PENDING bit is still set and new execution of
+ * a @work is not queued in a hope, that CPU#1 will eventually
+ * finish the queued @work. Meanwhile CPU#1 does not see
+ * event_indicated is set, because speculative LOAD was executed
+ * before actual STORE.
+ */
+ smp_mb();
}
static void clear_work_data(struct work_struct *work)