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Date: Fri, 17 Jul 2020 17:19:37 -0400
From: Rich Felker <dalias@...c.org>
To: Hydro Flask <hydroflask@...mail.com>,
Carlos O'Donell <carlos@...hat.com>,
Florian Weimer <fweimer@...hat.com>, musl@...ts.openwall.com
Subject: Re: Idea: futex() system call entry point
On Fri, Jul 17, 2020 at 11:10:50PM +0200, Szabolcs Nagy wrote:
> * Hydro Flask <hydroflask@...mail.com> [2020-07-17 11:57:36 -0700]:
> > On 2020-07-17 07:43, Carlos O'Donell wrote:
> > > On 7/17/20 5:21 AM, Szabolcs Nagy wrote:
> > > > * Hydro Flask <hydroflask@...mail.com> [2020-07-16 23:29:53 -0700]:
> > > > > On 2020-07-16 23:10, Florian Weimer wrote:
> > > > > > * Hydro Flask:
> > > > > >
> > > > > > > I have a project that implements an API that must be AS-safe.
> > > > > >
> > > > > > > Had the idea of using futex() but my other constraint is that the
> > > > > > > blocking call must also be a cancellation point.
> > > > > >
> > > > > > Cancellation points in signal handlers lead to asynchronous
> > > > > > cancellation. Are you sure that this is what you want?
> > > > >
> > > > > Yes I am aware of that. The caller is responsible for making
> > > > > sure it is safe
> > > > > to call the cancellation point in the signal handler per the
> > > > > recommendations
> > > > > in POSIX.
> > > >
> > > > how does the caller ensure that the interrupted
> > > > code is async cancel safe?
> > >
> > > I would also like to know that :-)
> > >
> > > Requiring AC-safety in the interrupted code is going
> > > to seriously limit what that code can call and do
> > > and indirectly what compiler and language implementation
> > > can even be used to implement that compiled code.
> >
> > There is a section in POSIX that covers exactly this, read the "Application
> > Usage" section of https://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_setcancelstate.html
> >
> > In general the user should ensure that cancellation is disabled one way or
> > another when the call is called from the signal handler, or that the call is
> > being done in a AC-safe region. There are a variety of ways to do this as
> > discussed in POSIX.
>
> it's possible to do this, but it's a rare requirement.
>
> futex is not a nice syscall to expose in c.
>
> currently there is disagreement about how to expose it:
> directly the linux api (which is variadic and not very
> typesafe) or separate calls for the useful operations
> (futex_wait, futex_wake, etc but the exact c api is
> less clear then).
>
> because of new time_t abi on 32bit targets, the timeout
> argument to futex is another reason to expose it in c
> instead of allowing users to use it via syscall (if they
> use the libc timespec type with the raw syscall that can
> be broken).
>
> in any case it's better to discuss this on libc-alpha
> since musl and glibc must expose the same api for it
> to be useful and it is harder to get this into glibc.
CC'ing libc-coord would also be appropriate for this, I think, even if
it is Linux-only and not relevant to the BSD etc folks there.
I do want to expose futex function but I don't want to end up with
something gratuitously incompatible/conflicting with what glibc ends
up doing.
Rich
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