[Gc] Re: Problem pthread_atfork

Andy Wingo wingo at pobox.com
Tue Mar 27 03:38:38 PST 2012

Hi Ivan,

On Tue 27 Mar 2012 10:23, Ivan Maidanski <ivmai at mail.ru> writes:

> Mon, 26 Mar 2012 18:55:43 +0200 Andy Wingo <wingo at pobox.com>:
>> On Mon 26 Mar 2012 14:59, Ivan Maidanski <ivmai at mail.ru> writes:
>> > 1. provide configure --enable/disable-handle-fork trigging [NO_]HANDLE_FORK
>> Sure, will do.
> Thanks. Although it is ok to pass it via CFLAGS, --enable-x is more uniform way.

Attached.  I leave it to you to regenerate the autofoo; I think we have
different versions.

> Another solution is to let the client decide whether fork should be handled by GC at runtime (in case the client rely on memory allocation after fork)?
> (E.g., export GC_fork_prepare/parent/child_proc).
> Yet more solution is to redirect fork to GC_fork (which calls prepare, fork, parent/child). Looks to me the best way.
> Your opinion?

I think that if libgc is to handle multithreaded fork, it needs to use
pthread_atfork and not some other API.  This is to ensure a consistent
locking order (at fork-time) between libgc, the libraries it uses
(e.g. libc), and the libraries that use it.  There can only be one
locking order, and thus one pthread_atfork.

>> If I may suggest, perhaps the right thing to do is not enable this code
>> by default, as it makes otherwise correct programs incorrect.  That is,
>> for a program that only calls signal-safe primitives after a
>> multi-threaded fork, this patch makes such a program depend on
>> unspecified behavior of their OS.
> I don't see: where is unspecified behavior if atfork is available? (we
> just adjust some global vars in GC at fork).

The client calls the following functions that are not async-signal-safe
from within the child atfork handler:


More broadly though, does it make sense to allocate memory at all after
a multithreaded fork?  The collector as a whole is not
async-signal-safe, so installing pthread_atfork() handlers does not
enable users to create correct programs.

I realize that the reality of the situation is that things will probably
work.  Libc installs atfork handlers for most of its data structures
(though not all).  But it's not something that is guaranteed to work, so
I'm not planning to make programs assuming that I can use the collector
after a multithreaded fork.



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