linux-kernel - Re: Stupid futex question

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Date:	Thu, 18 Feb 2010 20:08:42 -0800
From:	Darren Hart <dvhltc@...ibm.com>
To:	Valdis.Kletnieks@...edu
CC:	linux-kernel@...r.kernel.org
Subject: Re: Stupid futex question - 2.6.33-rc7-mmotm0210

Valdis.Kletnieks@...edu wrote:
> Kernel: x86_64 2.6.33-rc7-mmotm0210
> 
> I'm debugging a problem where pulseaudio is getting killed with a SIGKILL
> out of the blue.  It appears to be a problem where pulseaudio sets
> RLIMIT_RTTIME and the bound gets exceeded.  Analysis with 'top' shows
> a short spike of 96% system time, and the tail end of strace shows this:
> 
> [pid 25065] 01:50:20.371484 ioctl(28, USBDEVFS_CONTROL, 0x7fd3d76f630c) = 0 <0.000015>
> [pid 25065] 01:50:20.371548 ioctl(28, 0x40045532, 0x7fd3d76f636c) = 0 <0.000016>
> [pid 25065] 01:50:20.371611 open("/dev/snd/pcmC0D0p", O_RDWR|O_NONBLOCK|O_CLOEXEC <unfinished ...>
> [pid 25064] 01:50:20.371678 <... write resumed> ) = 8 <0.002104>
> [pid 25064] 01:50:20.371718 futex(0xc2ec00, FUTEX_WAIT_PRIVATE, 0, NULL <unfinished ...>

So it received the signal and the task waiting in 
sys_futex(FUTEX_WAIT_PRIVATE...) wakes up to handle the signal and 
return right? What do you see here that suggests the task is spinning 
inside the futex syscall?

This is a non-rt kernel right? So spinning could occur on the hash 
bucket lock if something else is really horked. That seems unlikely.

> [pid 25066] 01:50:21.408392 +++ killed by SIGKILL +++
> PANIC: handle_group_exit: 25066 leader 25064
> [pid 25065] 01:50:21.408442 +++ killed by SIGKILL +++
> PANIC: handle_group_exit: 25065 leader 25064
> 01:50:21.420354 +++ killed by SIGKILL +++
> 
> thread 25064 apparently gets gunned down due to RTTIME because it spent a whole
> second in a futex() call - is it reasonable for futex() to not return for that
> long?

Certainly if a corresponding FUTEX_WAKE op was not sent.

--
Darrem

> In other words - kernel bug because futex() should return, or pulseaudio bug
> for not understanding futex() can snooze a while?

> If a kernel bug, anybody got a better idea than nuking the RLIMIT_RTTIME call,
> waiting for it to repeat (takes between 1 minute and 1 hour or so), and
> whomping it a few times with sysrq-T?
> 


-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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