linux-kernel - Re: [RFC] Add implicit barriers to irqsave/restore class of functions

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Message-ID: <515E52A1.2080704@synopsys.com>
Date:	Fri, 5 Apr 2013 09:57:13 +0530
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Christian Ruppert <christian.ruppert@...lis.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Pierrick Hascoet <pierrick.hascoet@...lis.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC] Add implicit barriers to irqsave/restore class of functions

Hi Peter,

On 04/04/2013 09:43 PM, Peter Zijlstra wrote:
> -       : "cc");
> +       : "memory", "cc");
> That's not a memory barrier, that a memory clobber, aka a compiler
> barrier.

For the problem under consideration we indeed want a compiler barrier because the
error shows up due to a stale register which is live across a spinlock for
!PREEMPT_COUNT config.

However IMO doing this in irq save/restore macros is semantically incorrect since
those macros might be used elsewhere which don't need the compiler reload reg
semantics. Further per tglx' suggestion fixing preempt_* macros for !PREEMPT_COUNT
case would fix this independent of what arch is doing.

A patch to that effect was already posted to lists:
http://www.spinics.net/lists/kernel/msg1510885.html
Please let us know what you think.

Thx,
-Vineet

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