timerfd deadlock

timerfd deadlock

[Ken Brown]

While testing some new pipe code, I'm occasionally seeing a timerfd deadlock: 
The main thread is stuck in timerfd_tracker::dtor at timerfd.cc:354, waiting for 
the timerfd thread to close; and the timerfd thread is stuck at timerfd.cc:140, 
waiting to acquire a mutex that's held by timerfd_tracker::dtor.

I see this maybe 1 out of 4 times that I run 'make -j13' on a clean emacs source 
tree, and I see it only when my new pipe code is installed.  So I don't know if 
this is a bug in my code or if it's just coincidence that my code happens to 
trigger a timerfd bug.

Ken

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Re: timerfd deadlock

[Corinna Vinschen]

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On Jun  7 12:10, Ken Brown wrote:
> While testing some new pipe code, I'm occasionally seeing a timerfd deadlock: 
> The main thread is stuck in timerfd_tracker::dtor at timerfd.cc:354, waiting for 
> the timerfd thread to close; and the timerfd thread is stuck at timerfd.cc:140, 
> waiting to acquire a mutex that's held by timerfd_tracker::dtor.

Hmm, I had hoped my current code does not have a deadlock situation
anymore, but I was wrong, apparently.

Is there any chance to create an STC?


Thanks,
Corinna

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Corinna Vinschen
Cygwin Maintainer

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Re: timerfd deadlock

[Ken Brown]

On 6/11/2019 4:52 AM, Corinna Vinschen wrote:
> On Jun  7 12:10, Ken Brown wrote:
>> While testing some new pipe code, I'm occasionally seeing a timerfd deadlock:
>> The main thread is stuck in timerfd_tracker::dtor at timerfd.cc:354, waiting for
>> the timerfd thread to close; and the timerfd thread is stuck at timerfd.cc:140,
>> waiting to acquire a mutex that's held by timerfd_tracker::dtor.
> 
> Hmm, I had hoped my current code does not have a deadlock situation
> anymore, but I was wrong, apparently.
> 
> Is there any chance to create an STC?

I'll try.

Ken

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Re: timerfd deadlock

[Ken Brown]

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On 6/11/2019 4:52 AM, Corinna Vinschen wrote:
> On Jun  7 12:10, Ken Brown wrote:
>> While testing some new pipe code, I'm occasionally seeing a timerfd deadlock:
>> The main thread is stuck in timerfd_tracker::dtor at timerfd.cc:354, waiting for
>> the timerfd thread to close; and the timerfd thread is stuck at timerfd.cc:140,
>> waiting to acquire a mutex that's held by timerfd_tracker::dtor.
> 
> Hmm, I had hoped my current code does not have a deadlock situation
> anymore, but I was wrong, apparently.
> 
> Is there any chance to create an STC?

I haven't been able to create an STC.  The deadlock seems to depend on a rare 
timing fluke, and I've only seen it when running a parallel make on a clean 
emacs tree, with my new pipe code installed.

On the other hand, I think I have a simple fix, which I'll send to 
cygwin-patches.  With this patch, I ran the parallel make 15 times without 
getting the deadlock.  After reverting the patch, I got the deadlock on the 4th run.

Ken
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