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Message-ID: <1425922969.2475.394.camel@j-VirtualBox>
Subject: Re: [PATCH] locking/rwsem: Fix lock optimistic spinning when owner
 is not running
From: Jason Low <jason.low2@hp.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Davidlohr Bueso <dave@stgolabs.net>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Michel Lespinasse <walken@google.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Dave Jones <davej@codemonkey.org.uk>,
        Ming Lei <ming.lei@canonical.com>, jason.low2@hp.com
Date: Mon, 09 Mar 2015 10:42:49 -0700
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	 <20150307092126.GO23367@worktop.ger.corp.intel.com>
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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 :)