From: Szabolcs Nagy <nsz@port70.net>
To: Rich Felker <dalias@libc.org>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>,
Florian Weimer <fweimer@redhat.com>,
libc-alpha@sourceware.org
Subject: Re: [PATCH 2/2] manual: Document __libc_single_threaded
Date: Sat, 23 May 2020 08:49:41 +0200 [thread overview]
Message-ID: <20200523064941.GD26190@port70.net> (raw)
In-Reply-To: <20200522195326.GB1079@brightrain.aerifal.cx>
* 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.
next prev parent reply other threads:[~2020-05-23 6:49 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-20 18:12 [PATCH 0/2] Add __libc_single_threaded Florian Weimer
2020-05-20 18:12 ` [PATCH 1/2] Add the __libc_single_threaded variable Florian Weimer
2020-05-21 13:07 ` Szabolcs Nagy
2020-05-21 13:16 ` Florian Weimer
2020-05-21 13:26 ` Szabolcs Nagy
2020-05-20 18:12 ` [PATCH 2/2] manual: Document __libc_single_threaded Florian Weimer
2020-05-21 7:52 ` Michael Kerrisk (man-pages)
2020-05-21 12:17 ` Florian Weimer
2020-05-21 11:18 ` Szabolcs Nagy
2020-05-21 12:16 ` Florian Weimer
2020-05-21 12:50 ` Adhemerval Zanella
2020-05-21 13:09 ` Szabolcs Nagy
2020-05-21 13:15 ` Adhemerval Zanella
2020-05-21 13:30 ` Szabolcs Nagy
2020-05-21 13:44 ` Florian Weimer
2020-05-21 13:58 ` Adhemerval Zanella
2020-05-21 14:03 ` Florian Weimer
2020-05-22 10:01 ` Szabolcs Nagy
2020-05-22 10:05 ` Florian Weimer
2020-05-22 10:54 ` Szabolcs Nagy
2020-05-22 11:08 ` Florian Weimer
2020-05-22 15:07 ` Rich Felker
2020-05-22 16:14 ` Rich Felker
2020-05-22 16:36 ` Adhemerval Zanella
2020-05-22 17:02 ` Florian Weimer
2020-05-22 17:18 ` Florian Weimer
2020-05-22 17:28 ` Rich Felker
2020-05-22 17:40 ` Florian Weimer
2020-05-22 17:49 ` Rich Felker
2020-05-22 19:22 ` Szabolcs Nagy
2020-05-22 19:53 ` Rich Felker
2020-05-23 6:49 ` Szabolcs Nagy [this message]
2020-05-23 16:02 ` Rich Felker
2020-05-25 8:08 ` Florian Weimer
2020-05-25 8:08 ` Florian Weimer
2020-05-25 17:21 ` Rich Felker
2020-05-27 11:54 ` Florian Weimer
2020-05-27 15:36 ` Rich Felker
2020-06-03 15:00 ` Florian Weimer
2020-06-03 17:11 ` Rich Felker
2020-05-21 13:56 ` Adhemerval Zanella
2020-05-21 13:14 ` Florian Weimer
2020-05-21 14:32 ` Adhemerval Zanella
2020-06-03 15:48 ` Florian Weimer
2020-06-03 17:52 ` Adhemerval Zanella
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