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Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers

From Rich Felker <dalias@libc.org>
Newsgroups linux.kernel
Subject Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers
Date 2016-03-12 19:20 +0100
Message-ID <rbWiS-1L9-5@gated-at.bofh.it> (permalink)
References (5 earlier) <rbAV4-2r7-19@gated-at.bofh.it> <rbB4K-2v3-9@gated-at.bofh.it> <rbB4K-2v3-13@gated-at.bofh.it> <rbBep-2z4-3@gated-at.bofh.it> <rbVd8-VK-27@gated-at.bofh.it>
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On Sat, Mar 12, 2016 at 06:05:09PM +0100, Ingo Molnar wrote:
> 
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > On Fri, Mar 11, 2016 at 11:39 AM, Linus Torvalds
> > <torvalds@linux-foundation.org> wrote:
> > >
> > >  "An implementation may also mark other functions not specified in the
> > > standard as cancellation  points"
> > 
> > .. but that was from the Linux man-page. The open group has
> > 
> >  "An implementation shall not introduce cancellation points into any
> > other functions specified in this volume of POSIX.1-2008"
> > 
> > So yeah, it looks like there would need to be some way to filter things.
> > 
> > Oh well.
> 
> Is this really a big problem? Signals are asynchronous anyway, so if a C library 
> uses signal delivery for cancellation, it has to be ready to get the signal 
> delivered in the 'wrong' moment, for the wrong system call. The system call has to 
> be restarted in that case - or the interruption result has to be returned.

The signals used for cancellation are not interrupting; the handler is
installed with SA_RESTART. If cancellation is disabled when the
handler is invoked, it does nothing at all. Otherwise, it first
modifies the saved signal mask to leave itself block after it returns
(the reason why involves complex nested-signal corner cases you
probably don't want to know about). Then, if the signal handler
determines the interrupted context is at a cancellation point, it
rewrites the saved program counter to act on cancellation rather than
restarting the syscall. If not, it does nothing else.

> The _cancellation_ itself will then still be executed during the next suitable 
> cancellation point: which will be before doing the next cancellable system call 
> (or libc API).
> 
> So I think it can still all be made work with SA_SYNCHRONOUS.
> 
> It would only be a show stopper if Linux didn't cover all required system calls. 
> Covering _more_ system calls is not a problem AFAICS. But I might be missing 
> something ...

You're missing a lot.

Rich

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Thread

[RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Andy Lutomirski <luto@kernel.org> - 2016-03-09 02:30 +0100
  Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Ingo Molnar <mingo@kernel.org> - 2016-03-09 10:00 +0100
    Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Szabolcs Nagy <nsz@port70.net> - 2016-03-09 12:40 +0100
      Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Szabolcs Nagy <nsz@port70.net> - 2016-03-09 12:50 +0100
      Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Linus Torvalds <torvalds@linux-foundation.org> - 2016-03-09 20:50 +0100
        Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Andy Lutomirski <luto@amacapital.net> - 2016-03-09 22:00 +0100
          Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Linus Torvalds <torvalds@linux-foundation.org> - 2016-03-09 22:30 +0100
          Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Ingo Molnar <mingo@kernel.org> - 2016-03-10 12:00 +0100
        Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Rich Felker <dalias@libc.org> - 2016-03-10 04:40 +0100
          Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Ingo Molnar <mingo@kernel.org> - 2016-03-10 12:20 +0100
            Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Rich Felker <dalias@libc.org> - 2016-03-10 17:50 +0100
              Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Ingo Molnar <mingo@kernel.org> - 2016-03-10 19:10 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Rich Felker <dalias@libc.org> - 2016-03-11 00:30 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Szabolcs Nagy <nsz@port70.net> - 2016-03-11 01:20 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Rich Felker <dalias@libc.org> - 2016-03-11 01:50 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Andy Lutomirski <luto@amacapital.net> - 2016-03-11 02:20 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Szabolcs Nagy <nsz@port70.net> - 2016-03-11 02:40 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Rich Felker <dalias@libc.org> - 2016-03-11 03:00 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Szabolcs Nagy <nsz@port70.net> - 2016-03-11 03:00 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Ingo Molnar <mingo@kernel.org> - 2016-03-11 10:40 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Szabolcs Nagy <nsz@port70.net> - 2016-03-11 12:40 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Linus Torvalds <torvalds@linux-foundation.org> - 2016-03-11 20:30 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Andy Lutomirski <luto@amacapital.net> - 2016-03-11 20:40 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Linus Torvalds <torvalds@linux-foundation.org> - 2016-03-11 20:40 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Linus Torvalds <torvalds@linux-foundation.org> - 2016-03-11 20:50 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Ingo Molnar <mingo@kernel.org> - 2016-03-12 18:10 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Rich Felker <dalias@libc.org> - 2016-03-12 19:20 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Ingo Molnar <mingo@kernel.org> - 2016-03-12 18:10 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Rich Felker <dalias@libc.org> - 2016-03-12 19:10 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Ingo Molnar <mingo@kernel.org> - 2016-03-12 19:50 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Rich Felker <dalias@libc.org> - 2016-03-12 20:10 +0100
                Re: [musl] Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation  helpers Ingo Molnar <mingo@kernel.org> - 2016-03-12 18:10 +0100
  Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Andy Lutomirski <luto@amacapital.net> - 2016-03-09 19:00 +0100
    Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Andy Lutomirski <luto@amacapital.net> - 2016-03-09 22:30 +0100
      Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers Andy Lutomirski <luto@amacapital.net> - 2016-03-12 19:20 +0100

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