[PATCH 2/2] manual: Document __libc_single_threaded

[Szabolcs Nagy]

* Rich Felker <dalias@libc.org> [2020-05-22 15:53:26 -0400]:
> On Fri, May 22, 2020 at 08:22:50PM +0100, Szabolcs Nagy wrote:
> > so i think the safest implementation never sets
> > __libc_single_threaded back to true and second safest
> > is one that only sets it back to true in pthread_join
> > when there were no detached threads (or if using some
> > os api it can verify that there really is only one
> > kernel thread).
> > 
> > if we want to allow kernel entities to be around
> > but still tell the user when "as far as the libc
> > is concerned there is only one thread", then i
> > think __libc_single_threaded needs to be an extern
> > call (that acts as a compiler barrier).
> 
> Do relaxed order atomics provide a compiler barrier? If so, I think
> SYS_membarrier combined with one could be sufficient However using
> atomic type for a public interface like this is a bit of a
> compatibility thorn (requires compiler and std level supporting it).
> 
> Of course the same could be achieved with requirement to use a manual
> compiler barrier (dummy asm) but I think it's error-prone to assume
> the application would do it correctly.

no, relaxed atomics do not order unrelated memory accesses.

but a trick like you proposed for the musl internal need_locks
may work: __libc_single_threaded can go back to 1 when there
is only one thread left executing user code *and* that thread
called a libc function that has an acquire barrier.
(if thread exit has a release barrier then this should work)

this can allow earlier single threaded detection than only
considering pthread_join: e.g. stdio, malloc etc may do a
check and update the global after an acquire barrier, however
the compiler must not cache globals across libc calls for this
to work.