[PATCH 0/1] Fix deadlocks related to child processes

Corinna Vinschen corinna-cygwin@cygwin.com
Mon Oct 29 09:09:00 GMT 2018

On Oct 28 16:22, David McFarland wrote:
> For a long time I've been struggling with intermittent deadlocks and
> segfaults in emacs, seemingly related to invoking child processes.  I
> recently found a reliable way to reproduce one such deadlock:
> - install clean cygwin with: emacs-w32, clang
> - install flycheck from elpa
> - grab some non trivial C header e.g.:
>   $ cp /usr/include/stdio.h test.h
> - $ emacs -q test.h
> - start flycheck:
>   (progn (package-initialize)
>          (require 'flycheck)
>          (flycheck-mode))
> - add a character to the start of the first line
> - wait for flygheck to complete
> - repeat the last two steps until a deadlock occurs
> Breaking in gdb showed the main thread in `cygheap_protect.acquire ()`,
> from either _cfree or _cmalloc.  The thread holding the mutex was always
> "flasio", and it would either be continually segfaulting or looping in
> _cfree.
> I added some debug prints to cygheap and determined that it flasio was
> double-freeing an atomic_write_buf.  I added some more prints and found
> that it was two different fhandler objects freeing the same buffer.
> I then found that `fhandler_base_overlapped::copyto` would clear the
> buffer pointer after the copy, but none of the derived classes (pipe,
> fifo) did.
> Attached is a patch which clears the buffer pointers when copying pipes
> and fifos.
> It would probably be safer to move the buffer clear to a `operator=`,
> but I wanted to keep the patch as simple as possible and avoid
> refactoring.

Excellent detective work, thanks for the patch!  Pushed.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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