On Tue, 7 Apr 2015, Sebastian Andrzej Siewior wrote:
> futex_wake() wakes the waiter while holding the hb->lock. This leads to
> a similar double wake up on -RT if the waiter has a higher priority than
> the process perfroming the wake up.
Well, the non pi wakeup is designed in a way that the waiter side does
not take the hash bucket lock.
So if you observe that hb->lock contention and the resulting PI
boosting dance on RT, then it's not the wakeup/waiter exit path. That
must be something like this:
T1 T2
wakeup()
-->preemption
sys_exit(futex_wait)
...
sys_enter(futex_wake);
lock(hb->lock);
> -/*
> - * The hash bucket lock must be held when this is called.
> - * Afterwards, the futex_q must not be accessed.
> - */
The comment should stay here, because you are not allowed to access
the futex_q after q->lock_ptr has been set to NULL and that happens in
this function.
> -static void wake_futex(struct futex_q *q)
> +static struct task_struct *__wake_futex(struct futex_q *q)
> {
> struct task_struct *p = q->task;
>
> if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
> - return;
> + return NULL;
>
> /*
> * We set q->lock_ptr = NULL _before_ we wake up the task. If
> @@ -1117,6 +1113,19 @@ static void wake_futex(struct futex_q *q)
> */
> smp_wmb();
> q->lock_ptr = NULL;
> + return p;
...
> @@ -1256,14 +1266,23 @@ futex_wake(u32 __user *uaddr, unsigned int flags, int
> nr_wake, u32 bitset)
> if (!(this->bitset & bitset))
> continue;
>
> - wake_futex(this);
> + if (nr_wake == 1)
> + waiter = __wake_futex(this);
> + else
> + wake_futex(this);
> if (++ret >= nr_wake)
> break;
> }
> }
>
> spin_unlock(&hb->lock);
> +
> out_put_key:
> + if (waiter) {
> + wake_up_state(waiter, TASK_NORMAL);
> + put_task_struct(waiter);
> + }
This should go before out_put_key, because none of the other code
pathes which jump there can set waiter.
Thanks,
tglx
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