• Linus Torvalds's avatar
    Merge branch 'for-4.6-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq · b75a2bf8
    Linus Torvalds authored
    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
    b75a2bf8
workqueue.c 153 KB