Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3b)

Steven Rostedt
2010-01-12T16:27:48Z

On Tue, 2010-01-12 at 10:38 -0500, Mathieu Desnoyers wrote:

> > The UP-kernel case is handled by the #ifdef in sys_membarrier(), though
> > with a bit larger code footprint than the embedded guys would probably
> > prefer. (Or is the compiler smart enough to omit these function given no
> > calls to them? If not, recommend putting them under CONFIG_SMP #ifdef.)
>
> Hrm, that's a bit odd. I agree that UP systems could simply return
> -ENOSYS for sys_membarrier, but then I wonder how userland could
> distinguish between:
>
> - an old kernel not supporting sys_membarrier()
> -> in this case we need to use the smp_mb() fallback on the read-side
> and in synchronize_rcu().
> - a recent kernel supporting sys_membarrier(), CONFIG_SMP
> -> can use the barrier() on read-side, call sys_membarrier upon
> update.
> - a recent kernel supporting sys_membarrier, !CONFIG_SMP
> -> calls to sys_membarrier() are not required, nor is barrier().
>
> Or maybe we just postpone the userland smp_mb() question to another
> thread. This will eventually need to be addressed anyway. Maybe with a
> vgetmaxcpu() vsyscall.

I think Paul means to wrap all your other functions under the #ifdef.
What you have for sys_membarrier() is fine (just return 0 on UP) but you
also need to wrap the helper function above it under #ifdef CONFIG_SMP.
Don't rely on the compiler to optimize them out. If anything, you'll
probably get a bunch of warnings about static functions unused.

-- Steve