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    SubjectRe: [PATCH 1/2] workqueue: Catch more locking problems with flush_work()
    On Fri, Apr 20, 2012 at 01:18:19AM -0700, Stephen Boyd wrote:
    > On 4/20/2012 12:18 AM, Yong Zhang wrote:
    > > On Thu, Apr 19, 2012 at 11:26:47PM -0700, Stephen Boyd wrote:
    > >> complain in the case where the work is not queued. That case is not a
    > >> false positive. We will get a lockdep warning if the work is running
    > > IIRC, flush_work() is just a nop when a work is not queued nor running.
    >
    > Agreed, but it's better to always print a lockdep warning instead of
    > only when the deadlock is going to occur.

    It will IMHO.

    >
    > >
    > >> (when start_flush_work() returns true) solely with the
    > >> lock_map_acquire() on cwq->wq->lockdep_map.
    > > Yeah, that is the point we use lockdep to detect deadlock for workqueue.
    > >
    > > But when looking at start_flush_work(), for some case
    > > !(cwq->wq->saved_max_active == 1 || cwq->wq->flags & WQ_RESCUER),
    > > lock_map_acquire_read() is called, but recursive read is not added to
    > > the chain list. So when lock_map_acquire_read(&cwq->wq->lockdep_map)
    > > is called, deadlock will not be detected. I hope you don't hit that
    > > special case.
    >
    > Hmm. Originally I had what you suggested in my patch but I left it out
    > because I wasn't sure if it would cause false positives.
    > Do you see any
    > possibility for false positives?

    No, I don't. My test indeed show nothing (just build and boot).

    >I'll add it back in if not.

    It's great if you can try it :)

    Thanks,
    Yong


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