Bug#454183: libc6: deadlock in syslog() after fork

[Damian Pietras <daper@daper.net>]

Hi,

I've found this bug doing a research on a similar issue in my program
and think it's worth to add some information.

This is not a bug but a behaviour that conforms to the specification:

http://www.opengroup.org/onlinepubs/009695399/functions/pthread_atfork.html

In short: calling any function that is not async-safe (man 7 signal) in
child process after fork() call in a multi-threaded program has
undefined behaviour. The problem is with functions (own or system
library functions) that uses mutexes internally to protect global
resources. When you call fork at the moment when some mutexes are locked
you end up with a child that has only one thread and global data that
were modified in critical sections in other threads at the moment of fork()
are inconsistent. Also, mutexes are unusable in the child.

See the above pthread_atfork() description to find out more.

-- 
Damian Pietras

http://www.linuxprogrammingblog.com