dead lock in forked child

[Salvatore Sanfilippo]

Hi Jason,

thanks for your reply. Redis is known to surely allocate something in
the main thread before doing anything else, so this bug should not be
possible in our case and maybe we were observing something else? Or
actually Jemalloc < 3.0 had such an issue even if the main thread
allocate something (I found some mention related to fork+exec in the
3.0 changelog, we don't do exec btw.)? Thanks.

Cheers,
Salvatore

On Fri, Jun 1, 2012 at 8:31 PM, Jason Evans <jasone at canonware.com> wrote:
> On May 25, 2012, at 2:02 AM, Jokea wrote:
>
> I've found that a forked child runs into dead lock in a multithreaded
> application.
>
>
> jemalloc calls pthread_atfork() during initialization, but the test program
> does no allocation in the main thread before forking, and it launches
> threads that race with it.  It appears that one of those threads gets part
> way through allocator initialization before the fork occurs, which leaves
> the allocator in an inconsistent state (init_lock locked, but initialization
> incomplete).  The simple workaround is to allocate something before forking.
>
> A general fix in jemalloc is messy at best.  The possibilities that come to
> mind are 1) intercepting pthread_create() (or all fork-like system calls)
> much as the lazy locking code in mutex.c does and forcing allocator
> initialization, or 2) using a library initializer (function specified via
> compiler attribute to be run during library load) to force allocator
> initialization.  Both of these approaches are somewhat
> fragile; dlsym(RTLD_NEXT, …) can break if other libraries play similar
> games, and library initializers don't run early enough to prevent all
> possible failures.  In any case, I'll make a note to experiment with (2).
>
> Thanks,
> Jason
>
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-- 
Salvatore 'antirez' Sanfilippo
open source developer - VMware
http://invece.org

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because software is so complicated. Beauty is the ultimate defence
against complexity.
       — David Gelernter