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    Subject[PATCH 3.13 103/105] futex: Ensure get_futex_key_refs() always implies a barrier
    Date
    3.13.11.10 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    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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