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    SubjectRe: 3.4.4-rt13: btrfs + xfstests 006 = BOOM.. and a bonus rt_mutex deadlock report for absolutely free!
    On Fri, 13 Jul 2012, Mike Galbraith wrote:
    > On Fri, 2012-07-13 at 11:52 +0200, Thomas Gleixner wrote:
    > > On Fri, 13 Jul 2012, Mike Galbraith wrote:
    > > > On Thu, 2012-07-12 at 15:31 +0200, Thomas Gleixner wrote:
    > > > > Bingo, that makes it more likely that this is caused by copying w/o
    > > > > initializing the lock and then freeing the original structure.
    > > > >
    > > > > A quick check for memcpy finds that __btrfs_close_devices() does a
    > > > > memcpy of btrfs_device structs w/o initializing the lock in the new
    > > > > copy, but I have no idea whether that's the place we are looking for.
    > > >
    > > > Thanks a bunch Thomas. I doubt I would have ever figured out that lala
    > > > land resulted from _copying_ a lock. That's one I won't be forgetting
    > > > any time soon. Box not only survived a few thousand xfstests 006 runs,
    > > > dbench seemed disinterested in deadlocking virgin 3.0-rt.
    > >
    > > Cute. It think that the lock copying caused the deadlock problem as
    > > the list pointed to the wrong place, so we might have ended up with
    > > following down the wrong chain when walking the list as long as the
    > > original struct was not freed. That beast is freed under RCU so there
    > > could be a rcu read side critical section fiddling with the old lock
    > > and cause utter confusion.
    >
    > Virgin 3.0-rt appears to really be solid. But then it doesn't have
    > pesky rwlocks.

    Ah. So 3.0 is not having those rwlock thingies. Bummer.

    > > /me goes and writes a nastigram^W proper changelog
    > >
    > > > btrfs still locks up in my enterprise kernel, so I suppose I had better
    > > > plug your fix into 3.4-rt and see what happens, and go beat hell out of
    > > > virgin 3.0-rt again to be sure box really really survives dbench.
    > >
    > > A test against 3.4-rt sans enterprise mess might be nice as well.
    >
    > Enterprise is 3.0-stable with um 555 btrfs patches (oh dear).
    >
    > Virgin 3.4-rt and 3.2-rt deadlock gripe. Enterprise doesn't gripe, but
    > deadlocks, so I have another adventure in my future even if I figure out
    > wth to do about rwlocks.

    Hrmpf. /me goes to stare into fs/btrfs/ some more.


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