linux-kernel - Re: ERESTARTSYS escaping from sem

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Date:	Sat, 10 Oct 2009 12:08:03 -0700
From:	Jeremy Leibs <leibs@...lowgarage.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Blaise Gassend <blaise@...lowgarage.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Darren Hart <dvhltc@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: ERESTARTSYS escaping from sem_wait with RTLinux patch

Thomas, thanks for the quick reply.

On Sat, Oct 10, 2009 at 10:59 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> Blaise,
>
> On Sat, 10 Oct 2009, Blaise Gassend wrote:
>> 1) Where is the ERESTARTSYS being prevented from getting to user space?
>>
>> The only likely place I see for preventing ERESTARTSYS from escaping to
>> user space is in arch/*/kernel/signal*.c. However, I don't see how the
>> code there is being called if there no signal pending. Is that a path
>> for ERESTARTSYS to escape from the kernel?
>>
>> The following comment in kernel/futex.h in futex_wait makes me wonder if
>> two threads are getting marked as ERESTARTSYS. The first one to leave
>> the kernel processes the signal and restarts. The second one doesn't
>> have a signal to handle, so it returns to user space without getting
>> into signal*.c and wreaks havoc.
>>
>>     (...)
>>         /*
>>          * We expect signal_pending(current), but another thread may
>>          * have handled it for us already.
>>          */
>>         if (!abs_time)
>>                 return -ERESTARTSYS;
>>     (...)
>
> If the task is woken by a signal, then the task private flag
> TIF_SIGPENDING is set, but in case of a process wide signal the signal
> might have been handled by another thread of the same process before
> that thread reaches the signal handling code, but then ERESTARTSYS is
> handled gracefully. So you seem to trigger a code path which does not
> go through do_signal.
>
>> 2) Why would this be happening only with RT kernels?
>
> Slightly different timing and locking semantics.
>
>> 3) Any suggestions on the best place to patch/workaround this?
>>
>> My understanding is that if I was to treat ERESTARTSYS as an EAGAIN,
>> most applications would be perfectly happy. Would bad things happen if I
>> replaced the ERESTARTSYS in futex_wait with an EAGAIN?
>
> No workarounds please. We really want to know what's wrong.
>
> Two things to look at:
>
> 1) Does that happen with 2.6.31.2-rt13 as well ?

I am nearly certain we saw the problems with the newer kernel as well,
although that was back with a much less concise test and I've since
reinstalled over that machine in the process of trying a number of
different 32/64 hardy/jaunty configurations on different hardware.
I'll do a fresh install of that particular kernel with default
configuration options on our hardware and let you know a little later
today.

> 2) Add a check to the code path where ERESTARTSYS is returned:
>
>   if (!signal_pending(current))
>      printk(KERN_ERR ".....");
>
> If you can see that message then we'll look further. I'll give your
> script a test ride on my systems as well.
>
> Thanks,
>
>        tglx
>
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