HR timers prevent an itimer from generating EINTR?

[Mike Heffner <mikeh@fesnel.com>]

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?


Cheers,

Mike

-- 

   Mike Heffner   <mikeh@fesnel.com>