From: Catalin Marinas <catalin.marinas@xxxxxxx> 3.4.106-rc1 review patch. If anyone has any objections, please let me know. ------------------ commit 76835b0ebf8a7fe85beb03c75121419a7dec52f0 upstream. 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@xxxxxxx> Reported-by: Matteo Franchin <Matteo.Franchin@xxxxxxx> Fixes: b0c29f79ecea (futexes: Avoid taking the hb->lock if there's nothing to wake up) Acked-by: Davidlohr Bueso <dave@xxxxxxxxxxxx> Tested-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx> Cc: Darren Hart <dvhart@xxxxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Zefan Li <lizefan@xxxxxxxxxx> --- kernel/futex.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/kernel/futex.c b/kernel/futex.c index 41dfb18..fafc54f 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -212,6 +212,8 @@ static void drop_futex_key_refs(union futex_key *key) case FUT_OFF_MMSHARED: mmdrop(key->private.mm); break; + default: + smp_mb(); /* explicit MB (B) */ } } -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html