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Re: [RFC] mutex destruction (#13690): problem description and workarounds
- From: Rich Felker <dalias at libc dot org>
- To: Carlos O'Donell <carlos at redhat dot com>
- Cc: Torvald Riegel <triegel at redhat dot com>, GLIBC Devel <libc-alpha at sourceware dot org>
- Date: Thu, 4 Dec 2014 12:34:02 -0500
- Subject: Re: [RFC] mutex destruction (#13690): problem description and workarounds
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- References: <20141201153802 dot GV29621 at brightrain dot aerifal dot cx> <1417452125 dot 1771 dot 503 dot camel at triegel dot csb> <20141201170542 dot GY29621 at brightrain dot aerifal dot cx> <1417467150 dot 1771 dot 581 dot camel at triegel dot csb> <20141201212223 dot GZ29621 at brightrain dot aerifal dot cx> <1417553118 dot 3930 dot 14 dot camel at triegel dot csb> <20141202210316 dot GI29621 at brightrain dot aerifal dot cx> <547F17E3 dot 9060901 at redhat dot com> <1417703533 dot 22797 dot 16 dot camel at triegel dot csb> <5480807D dot 3040309 at redhat dot com>
On Thu, Dec 04, 2014 at 10:40:45AM -0500, Carlos O'Donell wrote:
> On 12/04/2014 09:32 AM, Torvald Riegel wrote:
> >> I agree. The conflation of EINTR for non-signal use is IMO going to be
> >> a design decision we regret in the future.
> > I'd rather see the fault in POSIX semantics, and it not making it clear
> > that signal handlers should do sem_post if they need to reliably
> > interrupt a sem_wait.
> If we are going to disallow a signal to interrupt sem_post we should just
> change the semantics, version the interface, and document that glibc no
> longer ever returns EINTR for sem_wait, and that the right way to interrupt
> it is with a signal handler that does sem_post.
> This prevents users from complaining that what they observe with strace
> and gdb is a signal arriving after the sem_wait, but not interrupting it.
> We can claim the user is looking under the hood, but that's what they do,
> and if we can possibly avoid those arguments we win. We know we're right,
> we know we don't want to allow timing to imply ordering, but we need time
> to educate developers (and that looking under the hood leads to non-obvious
> I really wish the kernel returned some other error code for woken up
> vs. signal. Is it not possible to get the kernel to distinguish these
> two? Am I forgetting something?
It *DOES*. It returns 0 for woken-up, and EINTR for
interrupted-by-signal. Torvald is proposing breaking the kernel by
adding non-signal situations under which it can return EINTR. The
kernel folks went to a lot of trouble to FIX all of the wrong
conditions under which EINTR could be returned (see man 7 signal) and
it would be a huge shame to undo that now.