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Message-ID: <ZqFpxCPjJ4vGwCCf@kib.kiev.ua>
Date: Wed, 24 Jul 2024 23:53:24 +0300
From: Konstantin Belousov <kostikbel@...il.com>
To: libc-coord@...ts.openwall.com
Cc: musl@...ts.openwall.com, libc-alpha@...rceware.org
Subject: Re: Making exit actually thread-safe
On Wed, Jul 24, 2024 at 03:09:48PM -0400, Rich Felker wrote:
> Inspired by Rust issue 126600, there seems to be renewed interest in
> the (non-) thread-safety of exit, namely the C (and copied in POSIX
> text) stipulation that calling exit "more than once" results in
> undefined behavior.
>
> This language predates threads and thus was certainly written with
> recursive calls (via atexit handlers) in mind as the way one might
> come to call exit "more than once".
>
> My view is that making calls to exit from multiple threads (one or
> more additional threads while the first caller is still acting on
> exit) undefined is a mistake and is harmful. There is a clear unique
> reasonable behavior for this situation: any remaining callers block
> until the first call to exit has finished, at which point they no
> longer exist. Leaving exit via longjmp, exceptions, etc. is undefined,
> so there are no concerns about what happens in such a situation. The
> additional calls to exit all behave as expected: the process
> terminates.
>
> Currently there is a glibc tracker item, #31997, and an Austin Group
> (POSIX) tracker item, #1845, open for this topic, as well as the
> original Rust tracker thread. I have participated in all of these, and
> would like to move this forward towards consensus on whether any new
> thread-safety requirement should be adopted, and if so, what the
> specific semantics should be.
>
> Links:
>
> https://github.com/rust-lang/rust/issues/126600
> https://sourceware.org/bugzilla/show_bug.cgi?id=31997
> https://austingroupbugs.net/view.php?id=1845
I do think that the wording in the austin group bug is reasonable, and
intend to do that for FreeBSD, https://reviews.freebsd.org/D46108
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