----- Original Message -----
> On Fri, Apr 17, 2015 at 11:06 AM, Mathieu Desnoyers
> <mathieu.desnoy...@efficios.com> wrote:
> > diff --git a/init/Kconfig b/init/Kconfig
> > index f5dbc6d..89bad6a 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1559,6 +1559,19 @@ config PCI_QUIRKS
> > bugs/quirks. Disable this only if your target machine is
> > unaffected by PCI quirks.
> >
> > +config MEMBARRIER
> > + bool "Enable membarrier() system call" if EXPERT
> > + default y
> > + depends on SMP
> > + help
> > + Enable the membarrier() system call that allows issuing memory
> > + barriers across all running threads, which can be used to
> > distribute
> > + the cost of user-space memory barriers asymmetrically by
> > transforming
> > + pairs of memory barriers into pairs consisting of membarrier()
> > and a
> > + compiler barrier.
> > +
> > + If unsure, say Y.
> > +
>
> 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.
Thanks,
Mathieu
> ...
>
> > +SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> > +{
> > + switch (cmd) {
> > + case MEMBARRIER_CMD_QUERY:
> > + return MEMBARRIER_CMD_BITMASK;
> > + case MEMBARRIER_CMD_SHARED:
> > + if (num_online_cpus() > 1)
> > + synchronize_sched();
> > + return 0;
> > + default:
> > + return -EINVAL;
> > + }
> > +}
>
>
>
> --
> Pranith
>
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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