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authorCatalin Marinas <catalin.marinas@arm.com>2014-10-17 17:38:49 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-10-30 09:38:24 -0700
commit19f4b01dbc75d117994c55d5e9cfa37a814f8b47 (patch)
tree6aa9653eb0f4e0458a91c47d6735bcb3f53a6f46
parent017ff97daa4a7892181a4dd315c657108419da0c (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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--kernel/futex.c2
1 files changed, 2 insertions, 0 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index 0b0dc02aabce..fda2950f2ce4 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -329,6 +329,8 @@ 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) */
}
}