Date: Thu, 24 Sep 2009 16:09:46 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Mike Heffner <mikeh@fesnel.com>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>, john stultz <johnstul@us.ibm.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Oleg Nesterov <oleg@redhat.com>
Subject: Re: HR timers prevent an itimer from generating EINTR?
Message-Id: <20090924160946.f54aa215.akpm@linux-foundation.org>
In-Reply-To: <4AA1860B.4030409@fesnel.com>
References: <4AA1860B.4030409@fesnel.com>
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(cc's added)

(it's a regression)

(it has a testcase!)

On Fri, 04 Sep 2009 17:26:35 -0400
Mike Heffner <mikeh@fesnel.com> wrote:

> Summary:
> 
> Mixing HR timers with itimers occasionally hides an EINTR from a 
> blocking syscall.
> 
> 
> Description:
> 
> In my test program I have a High Resolution timer firing every one 
> second (with SA_RESTART) and I set an itimer (without SA_RESTART) to 
> fire after three seconds. I then execute a blocking system call (flock 
> in this case) and expect the three second itimer to interrupt the system 
> call with EINTR. However, I frequently notice that the itimer will fire 
> but it will not interrupt the blocking system call. There appears to be 
> a race between the HR timer firing and the itimer firing. If I offset 
> the HR timer frequency by a half second, the itimer always interrupts 
> the system call.
> 
> Kernel version:
> 
> These kernels both demonstrate the condition:
> 
> 2.6.29.6-217.2.16.fc11.x86_64
> 	and
> 2.6.30.5-43.fc11.x86_64
> 
> 
> I do not see this condition on:
> 
> 2.6.18-53.el5
> 
> 
> Test program:
> 
> The following program illustrates this condition:
> 
> http://github.com/mheffner/scripts/commits/master/hrtimer_vs_itimer.c
> 
> 
> Is this behavior expected?
> 
> 

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