Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier

[Davidlohr Bueso]

On Fri, 2014-10-17 at 17:38 +0100, Catalin Marinas wrote:
> 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(). 

Good catch, glad I ran into this thread (my email recently changed).
Private process futex (PTHREAD_PROCESS_PRIVATE) have no reference on an
inode or mm so it would need the explicit barrier in those cases.

> 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).

Yeah missing wakeups are a strong sign of a problem with the
hb_waiters_pending() side.

> 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)
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: Davidlohr Bueso <davidlohr@xxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> 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>
> ---
>  kernel/futex.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 815d7af2ffe8..f3a3a071283c 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -343,6 +343,8 @@ 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) */
>  	}

Should we comment that this default is for the private futex case?
Otherwise:

Acked-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>

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