[PATCH 3/3] Refactor atfork handlers

[Florian Weimer]

On 02/20/2018 03:23 PM, Adhemerval Zanella wrote:

> Aside of the two scenarios (callbacks issuing fork/pthread_atfork), the only
> other scenario I see which might trigger a deadlock in this case is a signal
> handler issuing fork/pthread_atfork.
> 
> Former is BZ#4737 and my understanding is this should be a EWONTFIX due
> indication future POSIX specification to interpret fork as async-signal-unsafe
> (comment #19 and I am not sure if fork could be made async-signal-safe with
> ticket locks as Rich stated in comment #21).
> 
> Regarding later I think pthread_atfork is inherent async-signal-unsafe due
> it might return ENOMEM indicating it might allocate memory and our malloc
> is also async-signal-unsafe.
> 
> Am I missing a scenario you might be considering?

I looked at the acquired locks during fork, and you are right, the 
corner cases where a deadlock can happen in the upstream sources are 
quite obscure.  However, we do not currently acquire any ld.so locks, 
and I think I've seen patches which change that (because upstream is 
buggy and crash in the new child process).  If any ld.so locks are 
acquired around fork, then we have a lock ordering conflict in case an 
ELF constructor calls pthread_register_atfork (which is an extremely 
natural thing to do), like this:

Fork:

   pthread_register_atfork lock
     rtld load lock

dlopen:

   rtld load lock
     calling ELF constructors, and then:
       pthread_register_atfork lock

The older lock-free code avoids this.  You could do the same even with 
locks if you created a copy of the handler list on the heap.

Thanks,
Florian