Date: Wed, 5 Dec 2018 20:47:59 +0100 From: Markus Wichmann <nullplan@....net> To: musl@...ts.openwall.com Subject: Re: sem_wait and EINTR 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  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 . 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  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? > >  https://www.openwall.com/lists/musl/2018/02/24/3 >  https://github.com/libfuse/libfuse/blob/fuse-3.3.0/lib/fuse_loop_mt.c#L332 >  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
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