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Message-ID: <CABpewhFEqn1H7hLxxp53beNwzeAPhsu6n-C9gJnThYcLkc4wcw@mail.gmail.com>
Date: Fri, 7 Sep 2018 00:09:33 -0400
From: "Joseph C. Sible" <josephcsible@...il.com>
To: musl@...ts.openwall.com
Subject: Re: Compile-time flag to enable optional EINTR's?
> Can you discuss what you're trying to use EINTR for here?
It's for a Python program with several threads that are often waiting
on various locks, events, etc., all implemented by Python in terms of
POSIX semaphores. If a thread is just waiting for work to do, and I
trigger a graceful shutdown via a signal, I'd like it to give up
waiting for work immediately rather than holding up shutdown until it
times out doing nothing.
> Most uses of
> EINTR have fundamental race conditions -- if the signal arrives just
> moments before the syscall you hoped to interrupt, it won't get
> interrupted, and will block until some other event lets it proceed.
I acknowledge the race condition. It's too bad there's no "p" variant
of sem_timedwait(3) (thus of futex(2) as well) like there is of
pselect vs. select.
> If we did want to bring back EINTR for sem_[timed]wait, I think the
> right thing to do would be to look for a workaround for the underlying
> kernel bug, or some way to detect it and avoid honoring the EINTRs
> that happen on old buggy kernels. One thing that was once suggested,
> but I didn't really like it, was ignoring (retrying on) EINTR for
> sem_wait, where the caller may not be prepared for it to return
> without having decremented the semaphore value, but honoring it
> (failing) for sem_timedwait, where the caller has to be prepared for
> the possibility of failure (ETIMEDOUT) anyway. I didn't do much
> research into whether this would be conforming but I think it would.
Either one of these solutions sounds okay to me. I don't have any good
ideas on how to do the former, though.
Joseph C. Sible
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