Re: BUG: NTPL: waitpid() doesn't return?

[Matthias Urlichs]

Hi,

Linus Torvalds:
> 
> "Clearly" is not correct. What I bet happens is:
> 
> > 31346 execve("/usr/bin/apt-ftparchive", ["apt-ftparchive", "packages", "testing/all"], [/* 12 vars */] <unfinished ...>
> > 31346 <... execve resumed> )            = 0
> > 31346 exit_group(0)                     = ?
> > 31340 --- SIGCHLD (Child exited) @ 0 (0) ---
> > 31342 waitpid(31346,  <unfinished ...>
> 
> Notice how you do "waitpid()" for a _specific_ thread when you get a 
> SIGCHLD.
> 
> How the heck do you know _which_ thread it was that exited?

Please look again. The above trace clearly shows that #31346 has
exited, which is exactly the thread being waitpid()ed for.

What the program does is basically
- spawn four threads or so
- each thread forks off some process, and then waitpid()s for exactly
  that pid

... and all but the last waitpid() never returns even though all four
child processes have exit_group()ed.

> Rule: never EVER wait for a specific thread in a SIGCHLD handler. When you 
> get a SIGCHLD, your signal handler should just wait for "any thread". 

No SIGCHLD has been installed. (I checked the strace output.)

-- 
Matthias Urlichs     |     noris network AG     |     http://smurf.noris.de/
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