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Date: Fri, 17 Jul 2020 18:21:27 -0700
From: Hydro Flask <>
Cc: Carlos O'Donell <>, Florian Weimer <>
Subject: Re: Idea: futex() system call entry point

On 2020-07-17 16:30, Rich Felker wrote:
> On Fri, Jul 17, 2020 at 02:37:27PM -0700, Hydro Flask wrote:
>> Maybe a less complex suggestion is to expose a syscall_cp()
>> function, so you can get cancellation point functionality for any
>> system call. I actually quite like that option. How does that sound?
> In the specific case of futex waits, it's not clear to me that there's
> any side effect for which you need to know in the cancellation handler
> whether it occurred, so why can't you just enable async cancel around
> syscall() and disable it again after?

Oh I hadn't thought of that. That's actually a pretty good short-term 
solution. So you're saying:

     int fuxex_wait(int *uaddr, int val, const struct timespec *timeout)
             int old, ret;

             /* pthread_setcanceltype() automatically calls 
pthread_testcancel() if async is enabled */
             ret = pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS, 
             if (ret) {
                     errno = ret;
                     ret = -1;
             ret = syscall(SYS_futex, uaddr, FUTEX_WAIT, val, timeout);
             old = pthread_setcanceltype(old, &old);
             if (old) abort();
             return ret;

I think you're right that even if the futex call succeeds, it's fine to 
cancel since it does not mutate any meaningful observable state. I think 
that should satisfy all my requirements when doing this on musl. 
pthread_testcancel/pthread_setcanceltype should be AS-safe in musl if 
cancellation is disabled or the interrupted code is AC-safe.

That should likely also work in other libcs assuming a sane 
implementation of all the required functions involved. Thank you


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