GNU bug report logs - #32786
(system* ...) call somehow interferes with atomic-box on armv7l

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Message #20 received at 32786 <at> debbugs.gnu.org (full text, mbox):

From: Михаил Бахтерев <mob <at> k.imm.uran.ru>
To: Mark H Weaver <mhw <at> netris.org>
Cc: 32786 <at> debbugs.gnu.org
Subject: Re: bug#32786: (system* ...) call somehow interferes with atomic-box
 on armv7l
Date: Thu, 27 Sep 2018 11:18:16 +0500

Thanks for clarification! I'll be able to test the patch in a couple of
days.

- MB. Respectfully

On Thu, Sep 27, 2018 at 01:49:23AM -0400, Mark H Weaver wrote:
> Hi,
> 
> Thanks for the additional details.  I was able to reproduce the bug, and
> I believe I now see the problem.
> 
> 'atomic-box-compare-and-swap!' is implemented using
> 'atomic_compare_exchange_weak' (if available), but neglects to handle
> the case where 'atomic_compare_exchange_weak' may spuriously fail.  In
> that case, the box is left unchanged, although the observed value was
> equal to the expected value.
> 
> What's happening here is that the 'atomic-box-compare-and-swap!' in
> 'sleep-loop' fails spuriously, leaving the box in state #:accepted
> although it returns #:accepted as its result.  When the main loop
> discovers this, it changes the state to #:need-to-sleep, although the
> thread has already ended.
> 
> To confirm this hypothesis, I added a print statement to the main loop
> showing the state of the box that it observed during the protocol
> exchange, and indeed it sees #:accepted the first time it checks, and
> #:need-to-sleep in all later iterations.
> 
> I've attached a proposed patch that I hope will fix this problem.  If
> you'd be willing to test it, I'd be grateful, but otherwise I'll try to
> test it in the next week or so.  My access to armv7l boxes is somewhat
> limited.
> 
> Thanks for this report.
> 
>       Mark
> 
> 

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