Re: [PATCH 2/2] manual: Document __libc_single_threaded

[Florian Weimer via Libc-alpha <libc-alpha@sourceware.org>]

* Szabolcs Nagy:

> The 05/21/2020 10:15, Adhemerval Zanella wrote:
>> On 21/05/2020 10:09, Szabolcs Nagy wrote:
>> > The 05/21/2020 09:50, Adhemerval Zanella via Libc-alpha wrote:
>> >> On 20/05/2020 15:12, Florian Weimer via Libc-alpha wrote:
>> >>
>> >>> +@smallexample
>> >>> +if (__libc_single_threaded)
>> >>> +  atomic_fetch_add (&reference_count, 1, memory_order_relaxed);
>> >>> +else
>> >>> +  atomic_fetch_add (&reference_count, 1, memory_order_acq_rel);
>> >>> +@end smallexample
>> >>
>> >> Shouldn't the access to __libc_single_threaded be atomic itself
>> >> (at least with relaxed semantic)?
>> > 
>> > not if we guarantee that this object can only be
>> > written while the process is single threaded.
>> > 
>> > (e.g. an exiting detached thread cannot update it
>> > even if only one thread left.. because that may
>> > concurrently read it)
>> > 
>> 
>> OK, so I think we should outline that atomic operations are not required 
>> to acess this object and that once __libc_single_threaded is set 0 it will 
>> continue to indicate non-single thread even thread are jointed or detached 
>> thread finishes.
>
> what's wrong with pthread_join updating it?

It's tricky do it correctly if there are two remaining threads, one of
them the one being joined, the other one a detached thread.  A
straightforward implementation merely looking at __nptl_nthreads before
returning from pthread_join would not perform the required
synchronization on the detached thread exit.

Thanks,
Florian