Messages in this thread | | | Date | Tue, 12 Jan 2010 11:38:31 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3b) |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > 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.
Ah! Indeed! Thanks for helping me see the light. ;)
Mathieu
> > -- Steve > >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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