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

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Date:	Tue, 30 Oct 2012 14:45:52 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	anish kumar <anish198519851985@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...il.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

On Wed, 2012-10-31 at 03:33 +0900, anish kumar wrote:

> > CPU 0                                 CPU 1
> > 
> > data = something                     flags = IRQ_WORK_BUSY
> > smp_mb() (implicit with cmpxchg      smp_mb()
> >           on flags in claim)         execute_work (sees data from CPU
> > 0)
> > try to claim
> > 
> As I understand without the memory barrier proposed by you the situation
> would be as below:
> CPU 0                                 CPU 1
>  
> data = something                     flags = IRQ_WORK_BUSY
> smp_mb() (implicit with cmpxchg      execute_work (sees data from CPU 0)
>            on flags in claim)        
> _success_ in claiming and goes

Correct, because it would see the stale value of flags.

> ahead and execute the work(wrong?)
> 		                     cmpxchg cause flag to IRQ_WORK_BUSY
> 
> Now knows the flag==IRQ_WORK_BUSY
>                                 
> Am I right?

right.

> 
> Probably a stupid question.Why do we return the bool from irq_work_queue
> when no one bothers to check the return value?Wouldn't it be better if
> this function is void as used by the users of this function or am I
> looking at the wrong code.

Not a stupid question, as I was asking that to myself just earlier
today. But forgot to mention it as well. Especially, because it makes it
look like there's a bug in the code. Maybe someday someone will care if
their work was finished by itself, or some other CPU.

Probably should just nix the return value.

-- Steve


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