Re: rt_mutex_timed_lock() vs hrtimer_wakeup() race ?

Oleg Nesterov
2006-07-31T20:12:58Z

On 07/30, Steven Rostedt wrote:
>
> On Sun, 2006-07-30 at 08:36 +0400, Oleg Nesterov wrote:
> >
> > Another question, task_blocks_on_rt_mutex() does get_task_struct() and checks
> > owner->pi_blocked_on != NULL under owner->pi_lock. Why ? RT_MUTEX_HAS_WAITERS
> > bit is set, we are holding ->wait_lock, so the 'owner' can't go away until
> > we drop ->wait_lock.
>
> That's probably true that the owner can't disappear before we let go of
> the wait_lock, since the owner should not be disappearing while holding
> locks. But you are missing the point to why we are grabbing the
> pi_lock. We are preventing a race that can have us do unneeded work
> (see below).

Yes, I see. But ...

> ===================================================================
> --- linux-2.6.18-rc2.orig/kernel/rtmutex.c 2006-07-30 18:04:12.000000000 -0400
> +++ linux-2.6.18-rc2/kernel/rtmutex.c 2006-07-30 18:07:08.000000000 -0400
> @@ -433,25 +433,26 @@ static int task_blocks_on_rt_mutex(struc
> ...
> else if (debug_rt_mutex_detect_deadlock(waiter, detect_deadlock)) {
> spin_lock_irqsave(&owner->pi_lock, flags);
> - if (owner->pi_blocked_on) {
> + if (owner->pi_blocked_on)
> boost = 1;
> - /* gets dropped in rt_mutex_adjust_prio_chain()! */
> - get_task_struct(owner);
> - }
> spin_unlock_irqrestore(&owner->pi_lock, flags);

In that case ->pi_lock can't buy anything. With or without ->pi_lock this
check is equally racy, so spin_lock() only adds unneeded work?

Thanks!

Oleg.