[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 #13 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Rich Felker from comment #12)
> Your proposed solution is a lot more complex and invasive than mine; it's
> actually almost equivalent to the first-generation solution I used in musl
> for the problem, which turned out to be a bad idea, and thus got scrapped.

Experience is knowing what not to do :-)

> Most importantly, aside from being complex and ugly, it does not actually
> solve the worst problem, because this case is wrong:

I like it when we can talk concretely about use cases.

> "Cancellation delivered between syscall and clearing IN_SYSCALL: The
> SIGCANCEL handler will immediately cancel the thread."
> 
> In this case, unless the syscall failed with EINTR, you must not act on the
> cancellation request. Doing so is non-conforming to the requirement that the
> side effects upon cancellation match the side effects on EINTR (which is
> just a fancy way of saying, approximately, that cancellation can only take
> place if the syscall has already done its job, e.g. closing a fd,
> transferring some bytes, etc.).

I was not aware of this requirement. Is this written in POSIX or did this come
about from discussion with the Austin group around the problems with close()
being cancelled? Can you provide a reference to this?

> In addition, I suspect your solution has further flaws like what happens
> when you longjmp out of a signal handler that interrupted an AS-safe syscall
> which is a cancellation point. These issues can be solved with more
> complexity (extra work in longjmp), but the solution I've proposed is much
> simpler and has no corner cases that are difficult to handle.

Could you propose your design as a glibc wiki page so that we can look at it
and critique it? I'd be happy to adopt your solution, but I Want to review it
and put it through the same kind of use-cases as we discussed here.

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