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Date: Wed, 30 Apr 2014 18:50:14 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Jason Low <jason.low2@hp.com>
Cc: Davidlohr Bueso <davidlohr@hp.com>, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] rwsem: Support optimistic spinning
Message-ID: <20140430165014.GI11096@twins.programming.kicks-ass.net>
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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!