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Date: Wed, 27 Apr 2016 12:11:46 -0400 From: Tejun Heo <tj@...nel.org> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: linux-kernel@...r.kernel.org, Roman Pen <roman.penyaev@...fitbricks.com>, peter@...leysoftware.com, Jens Axboe <axboe@...nel.dk> Subject: [GIT PULL] workqueue fixes for v4.6-rc5 Hello, Linus. 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_rmb() 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. Thanks. The following changes since commit f7813ad5cbfd1fab2899914281b72a1ba0805c80: Merge tag 'for-linus-4.6' of git://git.code.sf.net/p/openipmi/linux-ipmi (2016-03-18 10:21:04 -0700) are available in the git repository at: git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git for-4.6-fixes for you to fetch changes up to 346c09f80459a3ad97df1816d6d606169a51001a: workqueue: fix ghost PENDING flag while doing MQ IO (2016-04-26 11:23:22 -0400) ---------------------------------------------------------------- Roman Pen (1): workqueue: fix ghost PENDING flag while doing MQ IO kernel/workqueue.c | 29 +++++++++++++++++++++++++++++ 1 file changed, 29 insertions(+) -- tejun
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