automoc4 processes lock again

[Max Brazhnikov]

On Mon, 9 May 2011 15:41:05 +0300, Kostik Belousov wrote:
> You did not supplied enough information.
> Which of the processes is parent, which is child ?
> Note that there are other threads in the pid 18636. What does they do ?

Here is backtraces from all threads http://people.freebsd.org/~makc/automoc4.bt
63373 is a parent now, 63374 is a child.

There were no related changes in Qt4 and automoc4 sources, probably my update from 8.2-PRERELEASE to STABLE a week ago triggered the issue.

> If you would allow me to make some guess, then I could assume that pid
> 18640 is the child. Note that the child is waiting for the pthread
> mutex locked which protects the stdio' FILE structure. Now, assume
> additionally that the parent had the FILE locked in one thread while
> another thread did the fork. Then, the child process would never be able
> to obtain the lock because the lock was acquired by the thread that
> exists no longer (in the child process, only the thread that called
> fork is duplicated).
> 
> In fact, I believe that you already reported a similar problem with
> malloc(3) some time ago. The root of the problem would be an undefined
> (and permitted by POSIX) behaviour of calling non-async signal safe
> functions in multithreaded process after fork.
> 
> For malloc(3), this can be argued to be a quality of the implementation
> issue, but there is no reason to specially handle random mutexes, even
> from libc. If the mutex was locked during the fork time, the protected
> data structure is arguably in the inconsistent state after the fork in
> the child.