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Subject: mq_send()/mq_receive() may never return if used from threads
To: cygwin AT cygwin DOT com
Message-ID: <838238ed-0bf4-9374-fb8a-bc6a47c512a9@t-online.de>
Date: Fri, 17 Jan 2025 16:56:59 +0100
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From: Christian Franke via Cygwin <cygwin AT cygwin DOT com>
Reply-To: cygwin AT cygwin DOT com
Cc: Christian Franke <Christian DOT Franke AT t-online DOT de>
Sender: "Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

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Found with 'stress-ng --workload ...':

If mq_receive is called on an empty queue and mq_send is called later 
from a different thread, both functions never return and signals 
(including SIGKILL) are no longer processed.

Testcase (attached):

$ uname -r
3.5.5-1.x86_64

$ gcc -o mqhang mqhang.c

$ ./mqhang
mq_send...
mq_send = 0
mq_receive...
mq_receive = 8
mq_receive...
mq_send...
[...hangs...]

Problem does not occur if mq_receive() is called from a fork()ed process 
instead of a thread.

-- 
Regards,
Christian


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