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

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Date:	Fri, 08 Aug 2014 12:03:20 -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 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. 

Thanks,
Davidlohr

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