Re: rt_mutex_timed_lock() vs hrtimer_wakeup() race ?

From: Thomas Gleixner
Date: Tue Aug 01 2006 - 08:49:43 EST

On Tue, 2006-08-01 at 08:07 -0400, Steven Rostedt wrote:
> > We hold lock->wait_lock. The owner of this lock can be blocked itself,
> > which makes it necessary to do the chain walk. The indicator is
> > owner->pi_blocked_on. This field is only protected by owner->pi_lock.
> >
> > If we look at this field outside of owner->pi_lock, then we might miss a
> > chain walk.
> >
> Actually Thomas, not counting the debug case, his patch wont miss a
> chain walk. That is because the boost is read _after_ the owner's prio
> is adjusted. So the only thing the lock is doing for us is to prevent
> us from walking the chain twice for the same lock grab. (btw. I'm
> looking at 2.6.18-rc2, and not the -rt patch, just to make things
> clear).

So what do we win, when we drop the lock before we check for boosting ?
In the worst case we do a redundant chain walk.


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