diff options
author | Catalin Marinas <catalin.marinas@arm.com> | 2014-10-17 17:38:49 +0100 |
---|---|---|
committer | Zefan Li <lizefan@huawei.com> | 2015-02-02 17:04:51 +0800 |
commit | 9e9aab5dbf6f13478cd996692ed9679af2404fc7 (patch) | |
tree | 3e8f726d264a7f0affc09df4609a9630493bc14f /kernel | |
parent | e8ab53a5d68c75722c8a2ad3b08a6798ed47d4dc (diff) |
futex: Ensure get_futex_key_refs() always implies a barrier
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@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)
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Tested-by: Mike Galbraith <umgwanakikbuti@gmail.com>
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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Zefan Li <lizefan@huawei.com>
Diffstat (limited to 'kernel')
-rw-r--r-- | kernel/futex.c | 2 |
1 files changed, 2 insertions, 0 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index 41dfb1866f95..fafc54fef885 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) */ } } |