From: Catalin Marinas <catalin.marinas@arm.com>
To: linux-kernel@vger.kernel.org
Cc: Matteo Franchin <Matteo.Franchin@arm.com>,
Davidlohr Bueso <davidlohr@hp.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Darren Hart <dvhart@linux.intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier
Date: Fri, 17 Oct 2014 17:38:49 +0100 [thread overview]
Message-ID: <1413563929-2664-1-git-send-email-catalin.marinas@arm.com> (raw)
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.
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) */
}
}
next reply other threads:[~2014-10-17 16:38 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-17 16:38 Catalin Marinas [this message]
2014-10-18 6:54 ` [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier Mike Galbraith
2014-10-18 7:09 ` Mike Galbraith
2014-10-18 15:28 ` Linus Torvalds
2014-10-18 16:15 ` Mike Galbraith
2014-10-18 7:33 ` Davidlohr Bueso
2014-10-18 19:58 ` Davidlohr Bueso
2014-10-18 20:50 ` Linus Torvalds
2014-10-19 2:16 ` Davidlohr Bueso
2014-10-20 9:11 ` Thomas Gleixner
2014-10-20 10:49 ` Catalin Marinas
2014-10-20 15:32 ` Linus Torvalds
2014-10-20 16:00 ` Catalin Marinas
2014-10-18 19:32 ` Darren Hart
2014-10-18 20:19 ` Davidlohr Bueso
2014-10-18 20:25 ` Darren Hart
2014-10-20 10:15 ` Catalin Marinas
2014-10-24 3:27 ` [PATCH] futex: Mention key referencing differences between shared and private futexes Davidlohr Bueso
2014-10-24 9:11 ` Catalin Marinas
2014-10-26 15:22 ` [tip:locking/urgent] " tip-bot for Davidlohr Bueso
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