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

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Date:	Wed, 30 Apr 2014 09:33:34 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
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 2:04 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Apr 28, 2014 at 03:09:01PM -0700, Davidlohr Bueso wrote:
>> +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
>> +{
>> +     int retval;
>
> And yet the return value is bool.
>
>> +     struct task_struct *owner;
>> +
>> +     rcu_read_lock();
>> +     owner = ACCESS_ONCE(sem->owner);
>> +
>> +     /* Spin only if active writer running */
>> +     if (owner)
>> +             retval = owner->on_cpu;
>> +     else {
>> +             /*
>> +              * When the owner is not set, the sem owner may have just
>> +              * acquired it and not set the owner yet, or the sem has
>> +              * been released, or reader active.
>> +              */
>> +             retval = false;
>> +     }
>
> And if you init the retval to false, you can leave this entire branch
> out.
>
>> +     rcu_read_unlock();
>> +
>> +     return retval;
>> +}
>
>
> Which yields the much shorter:
>
> 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;
> }
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