Message-ID: <5662182E.4050909@hpe.com>
Date: Fri, 04 Dec 2015 17:48:14 -0500
From: Waiman Long <waiman.long@hpe.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Peter Zijlstra <peterz@infradead.org>, Will Deacon <will.deacon@arm.com>,
        Ingo Molnar <mingo@kernel.org>, Oleg Nesterov <oleg@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>, Jonathan Corbet <corbet@lwn.net>,
        Michal Hocko <mhocko@kernel.org>, David Howells <dhowells@redhat.com>,
        Paul Turner <pjt@google.com>
Subject: Re: [PATCH 3/4] locking: Introduce smp_cond_acquire()
References: <20151203124010.627312076@infradead.org> <20151203124339.552838970@infradead.org> <20151203163725.GJ11337@arm.com> <20151203202627.GV17308@twins.programming.kicks-ass.net> <5661FCD0.60909@hpe.com> <CA+55aFzZA9EB3hFptSpdmeMOifeM5BWQGOW+ib7SLvyMTETzaA@mail.gmail.com>
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On 12/04/2015 05:05 PM, Linus Torvalds wrote:
> On Fri, Dec 4, 2015 at 12:51 PM, Waiman Long<waiman.long@hpe.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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