Re: [RFC][PATCH 2/3] futex: Reduce hash bucket lock contention

Peter Zijlstra
2011-09-16T12:35:10Z

On Wed, 2011-09-14 at 15:30 +0200, Peter Zijlstra wrote:
> @@ -964,6 +961,7 @@ futex_wake(u32 __user *uaddr, unsigned i
> struct futex_q *this, *next;
> struct plist_head *head;
> union futex_key key = FUTEX_KEY_INIT;
> + WAKE_LIST(wake_list);
> int ret;
>
> if (!bitset)
> @@ -988,7 +986,7 @@ futex_wake(u32 __user *uaddr, unsigned i
> if (!(this->bitset & bitset))
> continue;
>
> - wake_futex(this);
> + wake_futex(&wake_list, this);
> if (++ret >= nr_wake)
> break;
> }
> @@ -996,6 +994,8 @@ futex_wake(u32 __user *uaddr, unsigned i
>
> spin_unlock(&hb->lock);
> put_futex_key(&key);
> +
> + wake_up_list(&wake_list, TASK_NORMAL);
> out:
> return ret;
> }

So while initially I thought the sem patch was busted, it turns out this
one is.

Thomas managed to spot the race:

Task-0 Task-1

futex_wait()
queue_me()

futex_wake()
wake_list_add();
__unqueue_futex();
plist_del();
if (!plist_node_empty())
__set_current_state(TASK_RUNNNIG);

wake_up_list();
/* waking an already running task-0 */

I guess the biggest question is, do we care? Ideally everything should
be able to deal with spurious wakeups, although we generally try to
avoid them.