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    SubjectRe: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier
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    (fixes Davidlohr bounce)

    On Sat, 2014-10-18 at 08:54 +0200, Mike Galbraith wrote:
    > On Fri, 2014-10-17 at 17:38 +0100, Catalin Marinas wrote:
    > > Commit b0c29f79ecea (futexes: Avoid taking the hb->lock if there's
    > > nothing to wake up) changes the futex code to avoid taking a lock when
    > > there are no waiters. This code has been subsequently fixed in commit
    > > 11d4616bd07f (futex: revert back to the explicit waiter counting code).
    > > Both the original commit and the fix-up rely on get_futex_key_refs() to
    > > always imply a barrier.
    > >
    > > However, for private futexes, none of the cases in the switch statement
    > > of get_futex_key_refs() would be hit and the function completes without
    > > a memory barrier as required before checking the "waiters" in
    > > futex_wake() -> hb_waiters_pending(). The consequence is a race with a
    > > thread waiting on a futex on another CPU, allowing the waker thread to
    > > read "waiters == 0" while the waiter thread to have read "futex_val ==
    > > locked" (in kernel).
    > >
    > > Without this fix, the problem (user space deadlocks) can be seen with
    > > Android bionic's mutex implementation on an arm64 multi-cluster system.
    >
    > How 'bout that, you just triggered my "watch this pot" alarm.
    >
    > https://lkml.org/lkml/2014/10/8/406
    >
    > The hang I encountered with stockfish only ever happened on one specific
    > box. Linus/Thomas said it I was likely a problem with the futex usage,
    > but it suspiciously deterministic, so I put this on the "watch out for
    > further evidence" back burner.
    >
    > The barrier fixing up my problematic box smells a lot like evidence.
    >
    > > Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
    > > Reported-by: Matteo Franchin <Matteo.Franchin@arm.com>
    > > Fixes: b0c29f79ecea (futexes: Avoid taking the hb->lock if there's nothing to wake up)
    > > Cc: <stable@vger.kernel.org>
    > > Cc: Davidlohr Bueso <davidlohr@hp.com>
    > > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > > Cc: Darren Hart <dvhart@linux.intel.com>
    > > Cc: Thomas Gleixner <tglx@linutronix.de>
    > > Cc: Peter Zijlstra <peterz@infradead.org>
    > > Cc: Ingo Molnar <mingo@kernel.org>
    > > Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > > ---
    > > kernel/futex.c | 2 ++
    > > 1 file changed, 2 insertions(+)
    > >
    > > diff --git a/kernel/futex.c b/kernel/futex.c
    > > index 815d7af2ffe8..f3a3a071283c 100644
    > > --- a/kernel/futex.c
    > > +++ b/kernel/futex.c
    > > @@ -343,6 +343,8 @@ static void get_futex_key_refs(union futex_key *key)
    > > case FUT_OFF_MMSHARED:
    > > futex_get_mm(key); /* implies MB (B) */
    > > break;
    > > + default:
    > > + smp_mb(); /* explicit MB (B) */
    > > }
    > > }
    > >
    > > --
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    >




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