This is a note to let you know that I've just added the patch titled
futex: Ensure get_futex_key_refs() always implies a barrier
to the 3.16-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
futex-ensure-get_futex_key_refs-always-implies-a-barrier.patch
and it can be found in the queue-3.16 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 76835b0ebf8a7fe85beb03c75121419a7dec52f0 Mon Sep 17 00:00:00 2001
From: Catalin Marinas <catalin.marinas@xxxxxxx>
Date: Fri, 17 Oct 2014 17:38:49 +0100
Subject: futex: Ensure get_futex_key_refs() always implies a barrier
From: Catalin Marinas <catalin.marinas@xxxxxxx>
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: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/futex.c | 2 ++
1 file changed, 2 insertions(+)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -343,6 +343,8 @@ static void get_futex_key_refs(union fut
case FUT_OFF_MMSHARED:
futex_get_mm(key); /* implies MB (B) */
break;
+ default:
+ smp_mb(); /* explicit MB (B) */
}
}
Patches currently in stable-queue which might be from catalin.marinas@xxxxxxx are
queue-3.16/futex-ensure-get_futex_key_refs-always-implies-a-barrier.patch
queue-3.16/arm64-debug-don-t-re-enable-debug-exceptions-on-return-from-el1_dbg.patch
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