Re: [patch] threading fix, tid-2.5.47-A3

[Jamie Lokier <lk@tantalophile.demon.co.uk>]

Ulrich Drepper wrote:
> [...] It the very basic concept of always having a correct
> thread descriptor.  A process created with fork() from an MT app is
> different from one created with fork() from a single threaded app.  In
> the latter case the thread descriptors are not used and can contain
> garbage.  In the former case the descriptor better be correct all the
> time.  It basically the same problem as with setting the TLS "register".
>  clone() get the CLONE_TLS flag because the child and parent have
> (potentially) different TLS address and it is not possible to set the
> value before the fork() call (since the parent would have the wrong
> value for some time) nor after the fork() (since then there would be a
> window for a signal to arrive for an uninitialized thread).

Ok, I understand now.

    1. You need the child to have a valid thread descriptor immediately after
       fork(), and the parent's thread descriptor to be the same
       before and after fork().

    2. At all times, get_current_thread()->tid must return the current
       thread's tid in both the parent and child.

That is fine.  Just allocate a new TLS for the child, use
CLONE_SETTID|CLONE_CLEARTID|CLONE_SETTLS in your threaded fork(), and
pass the child's tid address (in the child's tls area).

It does require allocating a new TLS area on fork().  Is that a
problem?

cheers,
-- Jamie