linux-kernel - Re: [PATCH 2/2] irq_work: Fix racy IRQ_WORK

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Date:	Wed, 31 Oct 2012 14:32:33 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	anish kumar <anish198519851985@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH 2/2] irq_work: Fix racy IRQ_WORK_BUSY flag setting

2012/10/31 Steven Rostedt <rostedt@...dmis.org>:
> On Wed, 2012-10-31 at 20:04 +0900, anish kumar wrote:
>> nflags = 1 | 3
>> nflags = 2 | 3
>> In both cases the result would be same.If I am right then wouldn't this
>> operation be redundant?
>
> Right. Actually we could change the new loop to:
>
>         for (;;) {
>                 oflags = cmpxchg(&work->flags, flags, IRQ_WORK_FLAGS);
>                 if (oflags == flags)
>                         break;
>                 if (oflags & IRQ_WORK_PENDING)
>                         return false;
>                 flags = oflags;
>                 cpu_relax();
>         }

We could. But I wanted to keep the code able to handle new flags in
the future (such as IRQ_WORK_LAZY).

> Frederic,
>
> Would you like to add my explanation to your change log? You can add the
> entire thing, which I think would explain a lot to people.

It's indeed a very clear explanation. I'll put that in the changelog, thanks!
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