linux-kernel - Re: [PATCH 3/4] locking: Introduce smp_cond

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Date:	Fri, 04 Dec 2015 17:48:14 -0500
From:	Waiman Long <waiman.long@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Will Deacon <will.deacon@....com>,
	Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Jonathan Corbet <corbet@....net>,
	Michal Hocko <mhocko@...nel.org>,
	David Howells <dhowells@...hat.com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH 3/4] locking: Introduce smp_cond_acquire()

On 12/04/2015 05:05 PM, Linus Torvalds wrote:
> On Fri, Dec 4, 2015 at 12:51 PM, Waiman Long<waiman.long@....com>  wrote:
>> Will the following work?
> Are we trying to win some obfuscated C contest here?
>
> Just make it do something like (skipping backslashes to make it easier
> to type and read)
>
>   #define smp_cond_load_acquire(ptr, cond_expr) ({
>       typeof(*ptr) VAL;
>       for (;;) {
>            VAL = READ_ONCE(*ptr);
>            if (cond_expr) break;
>            cpu_relax();
>       }
>       smp_rmb();
>       VAL;
>    })
>
> and then you'd have it be
>
>      val = smp_cond_load_acquire(&lock->val.counter,
>                 !(VAL&  _Q_LOCKED_PENDING_MASK));
>
> which is at least halfway legible. Not some odd "fragments of
> expressions" interfaces unless absolutely required, please.

It is just some random thought that I have. I am not saying that it is 
the right way to go.

> Of course, I suspect we should not use READ_ONCE(), but some
> architecture-overridable version that just defaults to READ_ONCE().
> Same goes for that "smp_rmb()". Because maybe some architectures will
> just prefer an explicit acquire, and I suspect we do *not* want
> architectures having to recreate and override that crazy loop.
>
> How much does this all actually end up mattering, btw?
>
>                 Linus

I think what Will want to do is to provide an architecture specific 
replacement for the whole macro, not just part of it. So using READ_ONCE 
should be fine.

Cheers,
Longman
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