Messages in this thread | | | Subject | Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier | From | Mike Galbraith <> | Date | Sat, 18 Oct 2014 08:54:45 +0200 |
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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) */ > } > } > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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