linux-kernel - Re: [PATCH v2 1/7] locking/rwsem: check for active writer/spinner before wakeup

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Message-ID: <1407714603.7594.10.camel@buesod1.americas.hpqcorp.net>
Date:	Sun, 10 Aug 2014 16:50:03 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Waiman Long <waiman.long@...com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@...com>, aswin@...com
Subject: Re: [PATCH v2 1/7] locking/rwsem: check for active writer/spinner
 before wakeup

On Sun, 2014-08-10 at 17:41 -0400, Waiman Long wrote:
> On 08/08/2014 03:03 PM, Davidlohr Bueso wrote:
> > On Fri, 2014-08-08 at 14:30 -0400, Waiman Long wrote:
> >> I have 2 issues about this. First of all, the timing windows between
> >> atomic_set() and mutex_has_owner() check is really small, I doubt it
> >> will be that effective.
> > That is true, which is why I didn't bother showing any performance data
> > in the changelog. However, more important than any performance, avoiding
> > bogus wakeups is the _right_ thing to do when allowing lock stealing.
> >
> >> Secondly, I think you may need to call
> >> mutex_release() and debug_mutex_unlock() to make the debugging code
> >> work, but they seems to be called only under the wait_lock. So I think
> >> there is more work that need to be done before this patch is ready.
> > When !DEBUG both mutex_release() and debug_mutex_unlock() should be
> > no-ops. So this allows us to do the mutex_has_owner() check *without*
> > holding the wait_lock.
> >
> > When DEBUG is set, we don't even bother calling mutex_has_owner(), so
> > nothing changes.
> >
> > I don't understand your concern.
> 
> It is true I forgot the fact that MUTEX_SPIN_ON_OWNER is disabled when 
> DEBUG_MUTEX is on. However, mutex_release is controlled by the LOCKDEP 
> config variable which is independent of DEBUG_MUTEX. So it is still a 
> concern.

But afaict you cannot have LOCKDEP without enabling DEBUG_MUTEX (but not
necessarily vice-versa). Both are quite intertwined within other
debugging dependencies/options.

Thanks,
Davidlohr

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