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Message-ID: <aLpalVB2apB21r3g@voyager>
Date: Fri, 5 Sep 2025 05:35:49 +0200
From: Markus Wichmann <nullplan@....net>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: ABA problem in aio_suspend()
Am Thu, Sep 04, 2025 at 03:42:20PM -0400 schrieb Rich Felker:
> We generally don't do generation counters like this that are clearly
> logically incorrect and just rely on "it's going to wake before
> something that could plausibly happen happens".
You think oversleeping for *exactly* 2^31 updates is plausible?
> I think the "obvious"
> way to make it correct is not to allow keeping an existing waiter's
> tid in the futex, but always putting one's own tid there. However this
> can create high contention on the futex, perhaps unboundedly so with a
> lot of threads/cores. That's probably why I tried to just use an
> existing waiter tid.
>
This seems like it can create a livelocking situation, where two threads
calling aio_suspend() keep overwriting the other thread's TID before the
other thread can go to sleep.
> The obvious way you'd *want* to make it correct is with a condvar, but
> this can't be done because aio_suspend is required to be AS-safe, and
> thus cannot use locks, at least not without a lot of care. And it
> can't do anything like having the waiter join a userspace wait queue,
> since the AS-safety of aio_suspend means it's legal to longjmp out of
> it.
>
And the process-shared code for condvars uses a generation counter. So
there's another example of it in use.
Ciao,
Markus
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