Messages in this thread Patch in this message | | | From | Kamal Mostafa <> | Subject | [PATCH 3.13 103/105] futex: Ensure get_futex_key_refs() always implies a barrier | Date | Mon, 27 Oct 2014 11:57:18 -0700 |
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3.13.11.10 -stable review patch. If anyone has any objections, please let me know.
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From: Catalin Marinas <catalin.marinas@arm.com>
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: Kamal Mostafa <kamal@canonical.com> --- kernel/futex.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/kernel/futex.c b/kernel/futex.c index 4741b1f..2b1583e 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -194,6 +194,8 @@ static void get_futex_key_refs(union futex_key *key) case FUT_OFF_MMSHARED: atomic_inc(&key->private.mm->mm_count); break; + default: + smp_mb(); /* explicit MB (B) */ } } -- 1.9.1
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