Re: futex: clarification needed with drop_futex_key_refs and memory barriers

[Mike Galbraith]

(futex ordering pop-flare)

On Sat, 2016-03-26 at 10:56 -0500, Petros Koutoupis wrote:
> I stumbled on an interesting scenario which I am unable to fully explain and I
> was hoping to get some other opinions on why this would or wouldn't work.
> 
> In recent testing on a 48-core Haswell arch server, our multi-threaded user space
> application was utilizing 60% to 100% more CPU than on our smaller 24-core servers
> (running an identical load). After spending a considerable amount of time analyzing
> stack dumps and straces it became immediately apparent that those exact threads
> operating with the higher CPU utilization were off in futex land.
> 
> Shortly afterward I stumbled on commit 76835b0ebf8a7fe85beb03c75121419a7dec52f0
> (futex: Ensure get_futex_key_refs() always implies a barrier) which addressed the
> handling of private futexes and preventing a race condition by completing the
> function with a memory barrier. Now, I fully understand why this patch was implemented:
> to have a memory barrier before checking the "waiters." It makes sense. What doesn't
> make sense (so far) is when I apply the same patch to the drop counterpart,
> drop_futex_key_refs(), and the problem goes away. See the change and my notes below.
> 
> 
> --- linux/kernel/futex.c.orig   2016-03-25 19:45:08.169563263 -0500
> +++ linux/kernel/futex.c        2016-03-25 19:46:06.901562211 -0500
> @@ -438,11 +438,13 @@ static void drop_futex_key_refs(union fu
> 
>         switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
>         case FUT_OFF_INODE:
> -               iput(key->shared.inode);
> +               iput(key->shared.inode); /* implies smp_mb(); (B) */
>                 break;
>         case FUT_OFF_MMSHARED:
> -               mmdrop(key->private.mm);
> +               mmdrop(key->private.mm); /* implies smp_mb(); (B) */
>                 break;
> +       default:
> +               smp_mb(); /* explicit smp_mb(); (B) */
>         }
>  }
> 
> 
> The iput() and mmdrop() routines in the switch statement eventually use
> atomic_dec_and_test() which according to the Documentation/memory-barriers.txt
> implies an smp_mb() on each side of the actual operation. Notice that private
> futexes aren't handled by this (read below) this switch.
> 
> Now there is a wrapper put_futex_key() which is called in a few function as a
> way to clean up before before retrying, but in every case, and before it is
> called, a check is made to see if the futex is private and if so, retries at
> a more appropriate area of its respective function.
> 
> Now I have found two functions where this type of check/protection aren't made
> and I am curious as to if I stumbled on what could potentially lead to a race
> condition in a large SMP environment. Please refer to futex_wait() (called indirectly
> via unqueue_me()) and futex_requeue().
> 
> Any thoughts or opinions would be greatly appreciated. Thank you in advance.
> 
> --
> Petros
>