From: Markus Wichmann <nullplan@gmx.net>
To: musl@lists.openwall.com
Subject: Re: sem_wait and EINTR
Date: Wed, 5 Dec 2018 20:47:59 +0100 [thread overview]
Message-ID: <20181205194759.GA32233@voyager> (raw)
In-Reply-To: <20181205191605.72492698@orivej.orivej.org>
On Wed, Dec 05, 2018 at 07:16:05PM +0000, Orivej Desh wrote:
> Hi,
>
> musl differs from glibc in that it does not return from sem_wait() on EINTR.
> This mail [1] explains that this is useful to safeguard the software that does
> not check sem_wait() return code. However, since glibc does return EINTR, such
> bugs in the open source software seem to be eventually noticed and fixed.
>
> The musl behaviour has a disadvantage in that it makes sem_wait() difficult to
> interrupt (and delays the return from sem_timedwait() until the timeout), which
> is relied upon in particular by multithreaded fuse for breaking out of the
> main thread waiting loop [2]. IMHO the fuse implementation is sensible, since it
> looks better than the alternatives I could imagine, and I'm inclined to patch
> musl like this [3] to meet its expectations.
>
> Am I missing some implications? Would you reconsider returning from sem_wait()
> on EINTR? Could you suggest a good fix for fuse that does not change musl?
>
> [1] https://www.openwall.com/lists/musl/2018/02/24/3
> [2] https://github.com/libfuse/libfuse/blob/fuse-3.3.0/lib/fuse_loop_mt.c#L332
> [3] https://github.com/orivej/musl/commit/c4c38aaab4fc55c23669f7b81386b615609cc3e1
I wanted to suggest a reworking of libfuse to instead of waiting on a
semaphore maybe just wait on the actual thread. Then I read the source
of pthread_join() and noticed that it, too, would hang itself in a loop
it can't break out of due to EINTR.
Maybe the simplest solution would be to simply tell libfuse users to
call fuse_session_exit() from the SIGINT handler if they want this
behavior to be portable. If fuse_session_exit() is not
async-signal-safe, then handle SIGINT in another thread using
pthread_sigmask() and sigwaitinfo().
In any case, libfuse is relying on behavior not guarenteed by the
interface. The fact that a certain implementation of the interface
happens to provide that behavior is irrelevant.
On a practical note, I certainly never expected sem_wait() to be capable
of failing due to errors other than bad programming before. Coding that
in would make even simple things like the consumer-producer example by
Dijkstra look horrible!
Ciao,
Markus
next prev parent reply other threads:[~2018-12-05 19:47 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-05 19:16 Orivej Desh
2018-12-05 19:47 ` Markus Wichmann [this message]
2018-12-05 21:27 ` Ondřej Jirman
2018-12-05 21:58 ` Rich Felker
2018-12-06 2:43 ` Orivej Desh
2018-12-06 3:17 ` Rich Felker
2018-12-06 15:57 ` Markus Wichmann
2018-12-06 16:23 ` Rich Felker
2018-12-06 17:03 ` Markus Wichmann
2018-12-06 17:33 ` Markus Wichmann
2018-12-06 20:31 ` Orivej Desh
2018-12-09 2:51 ` Rich Felker
2018-12-09 6:50 ` Markus Wichmann
2018-12-12 0:32 ` Rich Felker
2018-12-12 5:15 ` Markus Wichmann
2018-12-14 19:45 ` Rich Felker
2018-12-15 9:45 ` Markus Wichmann
2018-12-05 22:03 ` Rich Felker
2018-12-06 2:43 ` Orivej Desh
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