(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.mari...@arm.com> > > Reported-by: Matteo Franchin <matteo.franc...@arm.com> > > Fixes: b0c29f79ecea (futexes: Avoid taking the hb->lock if there's nothing > > to wake up) > > Cc: <sta...@vger.kernel.org> > > Cc: Davidlohr Bueso <davidl...@hp.com> > > Cc: Linus Torvalds <torva...@linux-foundation.org> > > Cc: Darren Hart <dvh...@linux.intel.com> > > Cc: Thomas Gleixner <t...@linutronix.de> > > Cc: Peter Zijlstra <pet...@infradead.org> > > Cc: Ingo Molnar <mi...@kernel.org> > > Cc: Paul E. McKenney <paul...@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 majord...@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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