linux-kernel - Re: HR timers prevent an itimer from generating EINTR?

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Date:	Thu, 24 Sep 2009 16:09:46 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mike Heffner <mikeh@...nel.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	john stultz <johnstul@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: HR timers prevent an itimer from generating EINTR?

(cc's added)

(it's a regression)

(it has a testcase!)

On Fri, 04 Sep 2009 17:26:35 -0400
Mike Heffner <mikeh@...nel.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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