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

* Rich Felker <dalias@libc.org> wrote:

> On Sat, Mar 12, 2016 at 06:00:40PM +0100, Ingo Molnar wrote:
> >
> > * Linus Torvalds <torvalds@linux-foundation.org> wrote:
> >
> > > [...]
> > >
> > > Because if that's the case, I wonder if what you really want is not "sticky
> > > signals" as much as "synchronous signals" - ie the ability to say that a signal
> > > shouldn't ever interrupt in random places, but only at well-defined points
> > > (where a system call would be one such point - are there others?)
> >
> > Yes, I had similar 'deferred signal delivery' thoughts after having written up the
> > sticky signals approach, I just couldn't map all details of the semantics: see the
> > 'internal libc functions' problem below.
> >
> > If we can do this approach then there's another advantage as well: this way the C
> > library does not even have to poll for cancellation at syscall boundaries: i.e.
> > the regular system call fast path gets faster by 2-3 instructions as well.
>
> That is not a measurable benefit. You're talking about 2-3 cycles out of 10k or
> more cycles (these are heavy blocking syscalls not light things like SYS_time or
> SYS_getpid).

Huh? The list of 'must be' cancellable system calls includes key system calls
like:

open()
close()
read() variants
write() variants
poll()
select()

which can be and often are very lightweight. The list of 'may be cancellable'
system calls includes even more lightweight system calls.

I think you are confusing 'might block' with 'will block'. Most IO operations on a
modern kernel with modern hardware will not block!

You are scaring me ... :-(

Thanks,

Ingo

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