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

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Date:	Wed, 31 Oct 2012 09:51:56 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	anish kumar <anish198519851985@...il.com>
Cc:	Frederic Weisbecker <fweisbec@...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

On Wed, 2012-10-31 at 20:04 +0900, anish kumar wrote:

> > This now does:
> > 
> > 	CPU 1					CPU 2
> > 	-----					-----
> > 	(flags = 0)
> > 	cmpxchg(flags, 0, IRQ_WORK_FLAGS)
> > 	(flags = 3)
> > 	[...]

We can still add here:

						(fetch flags)

> > 	xchg(&flags, IRQ_WORK_BUSY)
> > 	(flags = 2)
> > 	func()
> > 						oflags = cmpxchg(&flags, flags, nflags);

Then the cmpxchg() would fail, and oflags would be 2

> > 						(sees flags = 2)
> > 						if (flags & IRQ_WORK_PENDING)

This should be:
						  if (oflags & IRQ_WORK_PENDING)


> > 							(not true)
> > 						(loop)						
> > 	cmpxchg(flags, 2, 0);
> > 	(flags = 2)

This should be:
	  (flags = 0)
as we described the previous cmpxchg() as failing, flags would still be
2 before this cmpxchg(), and this one would succeed.

-- Steve

> > 						flags = 3
> > 
> > 
> > 


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