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

From: Ingo Molnar
Date: Sat Mar 12 2016 - 13:48:56 EST

* Rich Felker <dalias@xxxxxxxx> wrote:

> On Sat, Mar 12, 2016 at 06:00:40PM +0100, Ingo Molnar wrote:
> >
> > * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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

read() variants
write() variants

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 ... :-(