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bug#18222: 24.3.92; fork handlers in gmalloc.c can lead to deadlock

From: Ken Brown
Subject: bug#18222: 24.3.92; fork handlers in gmalloc.c can lead to deadlock
Date: Fri, 08 Aug 2014 09:09:31 -0400
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0

malloc_enable_thread() in gmalloc.c calls pthread_atfork to set up fork handlers. There are a couple of problems with this, but the immediate reason for this bug report is a problem on Cygwin that was reported in the thread starting at


and continuing at


The issue is that the 'prepare' fork handler locks the pthread_mutexes prior to forking, and the ensuing processing of the fork command by the Cygwin DLL leads to a call to malloc in the same thread, resulting in deadlock. This is a long-standing problem, but it was masked until recently by the fact that pthread_mutexes on Cygwin were ERRORCHECK mutexes by default. As of Cygwin 1.7.31, pthread_mutexes are now NORMAL mutexes by default, so the problem has shown up.

A simple short-term workaround would be to explicitly set the mutexes to be ERRORCHECK or RECURSIVE mutexes on Cygwin, thereby restoring the previous behavior. But this does not seem like the right long-term solution, for the reasons explained here:


I know nothing about this other than what I learned from the two messages above, so I would appreciate some guidance.


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