linux-kernel - Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner is not running

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Date:	Mon, 09 Mar 2015 10:42:49 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Michel Lespinasse <walken@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Ming Lei <ming.lei@...onical.com>, jason.low2@...com
Subject: Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner
 is not running

On Sat, 2015-03-07 at 10:21 +0100, Peter Zijlstra wrote:
> On Fri, Mar 06, 2015 at 11:45:31PM -0800, Jason Low wrote:
> >  static noinline
> >  bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
> >  {
> >  	long count;
> >  
> >  	rcu_read_lock();
> > +	while (sem->owner == owner) {
> > +		/*
> > +		 * Ensure we emit the owner->on_cpu, dereference _after_
> > +		 * checking sem->owner still matches owner, if that fails,
> > +		 * owner might point to free()d memory, if it still matches,
> > +		 * the rcu_read_lock() ensures the memory stays valid.
> > +		 */
> > +		barrier();
> > +
> > +		/* abort spinning when need_resched or owner is not running */
> > +		if (!owner->on_cpu || need_resched()) {
> >  			rcu_read_unlock();
> >  			return false;
> >  		}
> 
> Thanks, looks good; do we want to change the mutex code (again) to more
> closely resemble this too? It still has the while (true) instead of the
> while(lock->owner != owner).

Yeah, I wondered about the same thing, though I wasn't sure if you
wanted an additional patch just for that change.

Guess you answered it for me :)


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