[Bug nptl/12683] Race conditions in pthread cancellation

["carlos at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=12683

--- Comment #15 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Rich Felker from comment #14)

Thanks for the recap.

> Anyway, this race condition is the whole matter at hand here. The two
> possibilities allowed (again, due to the limitations imposed by the first
> paragrah) are acting on the event while leaving cancellation pending, or
> acting on cancellation while leaving the event pending. But glibc also has a
> race window where it can act on both the event, producing side effects
> (because the kernel already has), and act on cancellation. This makes it
> non-conforming and makes it impossible to use cancellation safely.

So does this imply that the cancellation *must* happen at some point after
errno is known? Thus if a cancellation arrives and we're already in the syscall
there is nothing to do but let the syscall return and let the syscall wrapper
handle the cancellation. That seems reasonable to me.

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