Re: sigaction, fork, malloc, and futex

[chris@scary.beasts.org]

Hi-

On Tue, 4 May 2004, Steve Beaty wrote:

>
> 	anyone have a clue on this one?  we set up a signal handler, create
> 	a child that sends that signal, and have the signal handler fork
> 	another child.  if there is a malloc(), the second child gets stuck
> 	in a futex(); without the malloc(), no problem.  2.4.20-30.9
> 	kernel.  straces at the end.  any help would be appreciated.
> 	thanks!

Your signal handler function is illegally calling non-reentrant functions.
The *printf() family of functions are going to need to call malloc() to
allocate buffers. malloc() cannot be re-entered.

So specifically your deadlock sequence is:

Parent:
fork()
fprintf()
-> malloc()
   -> take a malloc() lock
(Child schedules and sends SIGALRM at this point)
SIGALRM:
fprintf()
-> malloc()
   -> try to take a malloc() lock
      -> deadlock, lock is already taken and will never be released!

Modern glibc / kernel combinations which use futexes in the malloc code
really seem to expose this race.

Cheers
Chris