Messages in this thread | | | Subject | Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3b) | From | Steven Rostedt <> | Date | Tue, 12 Jan 2010 11:27:39 -0500 |
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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
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