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.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/
