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Date: Sun, 5 Apr 2015 16:23:14 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Resuming work on new semaphore
On Sun, Apr 05, 2015 at 11:03:34PM +0300, Alexander Monakov wrote:
> On Sun, 5 Apr 2015, Rich Felker wrote:
> > 1. Thread A enters sem_wait.
> > 2. Thread B observes thread A in sem_wait via failed sem_trywait.
>
> Hm, I don't see how that can be achieved. As a result I'm afraid I didn't
> fully understand your example.
Indeed I was wrong about that, so I agree the whole scenario may fall
apart. Only sem_getvalue could show this, and only if it returns -1
rather than 0. So returning negative values from sem_getvalue seems
like a very bad idea -- it puts difficult- or impossible-to-satisfy
additional constraints on the implementation.
> > > Well we can make sem_getvalue return val[0]+val[1] instead... ;)
> >
> > That just makes the new implementation look like the old one, no? :-)
>
> Can't be bad if it behaves the same but works a bit faster.
> Apropos, like I've said on IRC, looks like there's "semaphore uncertainty
> principle": that formal semaphore value is between val[0] and (val[0] +/-
> val[1]) (clamped to 0 as needed). It seems you can either do your hack and
> pretend that there are never any waiters, or try to faithfully count waiters
> in sem_getvalue, but then also reveal that sometimes the implementation works
> by stealing a post. I believe you could argue that the latter is explicitely
> disallowed by the spec.
Yes, I think I agree.
> By the way, I think there's an interesting interplay with cancellation.
> Consider the following. Thread B does "return sem_wait(sem);". Thread A does:
>
> pthread_cancel(thread_B);
> sem_post(sem);
> sem_getvalue(sem);
>
> If it observes semaphore value as 1 it follows that thread B has not become a
> waiter yet, and since it must have cancellation already pending, it may not
> consume the post. And yet if thread B is already futex-waiting in sem_wait,
> consuming the post takes priority over acting on cancellation. So if then
> thread A does
>
> pthread_join(thread_B);
> sem_getvalue(sem);
>
> and gets value of 0, it sees a contradiction. And return value from
> pthread_join will indicate that thread_B exited normally rather than was
> cancelled.
So the contradiction you claim exists is that cancellation happened
before the post, and thus thread B can't act on the post when it
didn't act on cancellation? I don't think that follows from the rules
of cancellation. The relevant text is:
"Whenever a thread has cancelability enabled and a cancellation
request has been made with that thread as the target, and the
thread then calls any function that is a cancellation point (such
as pthread_testcancel() or read()), the cancellation request shall
be acted upon before the function."
So if cancellation was pending _before_ the call to sem_wait, then
sem_wait has to honor it. But there is no requirement that entry to
the sem_wait function be "atomic" with becoming a waiter on the
semaphore, and of course this is impossible to satisfy or even
specify. So it's totally legal to have the sequence:
1. Thread B enters sem_wait.
2. Thread B observes that cancellation was not already pending.
3. Thread A sends cancellation request.
4. Thread A sends post.
5. Thread B receives both, and chooses to act on the post per this
text:
"It is unspecified whether the cancellation request is acted upon
or whether the cancellation request remains pending and the thread
resumes normal execution if:
- The thread is suspended at a cancellation point and the event for
which it is waiting occurs
- A specified timeout expired
before the cancellation request is acted upon."
Here, the event for which it was waiting (the post) clearly occurs.
> And on the contrary, if you make acting on cancellation/timeout take priority,
> you can observe semaphore value increasing when waiters leave the wait on
> error path without consuming the post.
Yes obviously that is not possible.
Rich
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