linux-kernel - Re: [PATCH v2] rwsem: Support optimistic spinning

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Message-ID: <20140430165014.GI11096@twins.programming.kicks-ass.net>
Date:	Wed, 30 Apr 2014 18:50:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	Davidlohr Bueso <davidlohr@...com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...aro.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E.McKenney" <paulmck@...ux.vnet.ibm.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] rwsem: Support optimistic spinning

On Wed, Apr 30, 2014 at 09:33:34AM -0700, Jason Low wrote:
> > static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> > {
> >         struct task_struct *owner;
> >         bool on_cpu = false;
> 
> Wouldn't we want to initialize on_cpu = true. For the !owner case, I
> would expect that we want to spin for the lock.
> 
> >         rcu_read_lock();
> >         owner = ACCESS_ONCE(sem->owner);
> >         if (owner)
> >                 on_cpu = owner->on_cpu;
> >         rcu_read_unlock();
> >
> >         return on_cpu;
> > }

That would indeed be in line with that the mutex code does. Indeed!
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