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

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Message-ID: <20151203211651.GQ11639@twins.programming.kicks-ass.net>
Date:	Thu, 3 Dec 2015 22:16:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	mingo@...nel.org, oleg@...hat.com, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, boqun.feng@...il.com, corbet@....net,
	mhocko@...nel.org, dhowells@...hat.com,
	torvalds@...ux-foundation.org, waiman.long@....com, pjt@...gle.com
Subject: Re: [PATCH 3/4] locking: Introduce smp_cond_acquire()

On Thu, Dec 03, 2015 at 09:26:27PM +0100, Peter Zijlstra wrote:
> The best we can do is hardcode a variable name; maybe something like:
> 
> #define smp_cond_acquire(ptr, expr) do {			\
> 	typeof(*ptr) val;					\
> 	while ((val = READ_ONCE(*ptr)), expr)			\
> 		cpu_relax();					\
> 	smp_rmb(); /* ctrl + rmb := acquire */			\
> } while (0)
> 
> Which would let us write:
> 
>   smp_cond_acquire(&lock->val.counter, !(val & _Q_LOCKED_PENDING_MASK));

So I'm thinking that might still be hard to use for you if the LDXR+WFE
has the regular LL/SC constraint of not allowing other loads in.
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