linux-kernel - Re: [RFC][PATCH 05/17] x86: Optimize arch_spin_unlock

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Date:	Fri, 24 Dec 2010 10:26:05 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <oleg@...hat.com>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>,
	Yong Zhang <yong.zhang0@...il.com>,
	linux-kernel@...r.kernel.org, Nick Piggin <npiggin@...nel.dk>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [RFC][PATCH 05/17] x86: Optimize arch_spin_unlock_wait()

On Fri, Dec 24, 2010 at 4:23 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> Only wait for the current holder to release the lock.
>
> spin_unlock_wait() can only be about the current holder, since
> completion of this function is inherently racy with new contenders.
> Therefore, there is no reason to wait until the lock is completely
> unlocked.

Is there really any reason for this patch? I'd rather keep the simpler
and more straightforward code unless you have actual numbers.

> +static inline void __ticket_spin_unlock_wait(arch_spinlock_t *lock)
> +{
> +       int tmp = ACCESS_ONCE(lock->slock);
> +
> +       if (!(((tmp >> TICKET_SHIFT) ^ tmp) & TICKET_MASK))
> +               return; /* not locked */
> +
> +       /* wait until the current lock holder goes away */
> +       while ((lock->slock & TICKET_MASK) == (tmp & TICKET_MASK))
> +               cpu_relax();
>  }

Also, the above is just ugly. You've lost the ACCESS_ONCE() on the
lock access, and it's using another model of masking than the regular
one. Both of which may be intentional (maybe you are _trying_ to get
the compiler to just load the low bytes and avoid the 'and'), but the
whole open-coding of the logic - twice, and with different looking
masking - just makes my skin itch.

                                  Linus
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