linux-kernel - Re: [PATCH 0/2] irq

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Date:	Tue, 30 Oct 2012 11:52:30 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	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 0/2] irq_work: A couple fixes

On Tue, 2012-10-30 at 16:34 +0100, Frederic Weisbecker wrote:
> Hi,

> And I still wonder if cpu_relax() is enough to prevent the compiler
> from correctly reloading work->flags in irq_work_sync() loop.
> Do we need ACCESS_ONCE()?

You mean this loop:

       flags = work->flags & ~IRQ_WORK_PENDING;
       for (;;) {
               nflags = flags | IRQ_WORK_FLAGS;
               oflags = cmpxchg(&work->flags, flags, nflags);
               if (oflags == flags)
                       break;
               if (oflags & IRQ_WORK_PENDING)
                       return false;
               flags = oflags;
               cpu_relax();
       }

After the first loading of work->flags, you are worried about the
&work->flags in the cmpxchg()?  The cmpxchg() will handle that itself. I
don't see any place that a ACCESS_ONCE() is required here. The cmpxchg()
acts on the address of work->flags, the compiler isn't involved with the
value at that address.

-- Steve


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