Re: [PATCH v16] sys_membarrier(): system-wide memory barrier (generic, x86)

[Stephen Hemminger <stephen@networkplumber.org>]

On Wed, 22 Apr 2015 20:37:08 -0400
Pranith Kumar <bobby.prani@gmail.com> wrote:

> >> I understand why this syscall makes sense on SMP only, but you are
> >> anyways checking num_online_cpus() and returning if it is only one. Is
> >> this limitation necessary then? How do !SMP systems handle this
> >> syscall? (I am guessing glibc wrapper?)  
> >
> > For !SMP, this system call is not implemented (returns -ENOSYS).
> > Userspace libs are expected to query sysconf(_SC_NPROCESSORS_CONF)
> > and check whether the system supports multiprocessor at all. If
> > only a single processor is supported by the kernel, then userspace
> > can skip the calls to sys_membarrier altogether, because they are
> > not even needed.
> >
> > Do you think this kind of information belongs in a man page ?
> >
> > Should we instead just implement the system call in !SMP, and
> > return 0 without any side-effect ? This would be a bit inefficient
> > to let userspace call a system call that has no effect whatsoever.
> >  
> 
> Are there any other SMP-only system calls like this? I am not really
> sure what is the right way but documenting it would be good.


The syscall should just return 0.
Let the application not worry about how many CPU's are present