Messages in this thread | | | Date | Tue, 13 Sep 2016 12:05:42 +1000 | From | Nicholas Piggin <> | Subject | Re: Question on smp_mb__before_spinlock |
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On Mon, 12 Sep 2016 14:54:03 +0200 Peter Zijlstra <peterz@infradead.org> wrote:
> On Mon, Sep 12, 2016 at 12:27:08PM +1000, Nicholas Piggin wrote: > > On Wed, 7 Sep 2016 15:23:54 +0200 > > Peter Zijlstra <peterz@infradead.org> wrote: > > > > Interesting idea.. > > > > > > So I'm not a fan of that raw_spin_lock wrapper, since that would end up > > > with a lot more boiler-plate code than just the one extra barrier. > > > > #ifndef sched_ctxsw_raw_spin_lock > > #define sched_ctxsw_raw_spin_lock(lock) raw_spin_lock(lock) > > #endif > > > > #define sched_ctxsw_raw_spin_lock(lock) do { smp_mb() ; raw_spin_lock(lock); } while (0) > > I was thinking you wanted to avoid the lwsync in arch_spin_lock() > entirely, at which point you'll grow more layers. Because then you get > an arch_spin_lock_mb() or something and then you'll have to do the > raw_spin_lock wrappery for that. > > Or am I missing the point of having the raw_spin_lock wrapper, as > opposed to the extra barrier after it? > > Afaict the benefit of having that wrapper is so you can avoid issuing > multiple barriers.
Oh you could do that too yes. But it's all going to be in arch/powerpc, so I don't know if layers would be much problem.
I was thinking to avoid the hwsync inside the critical section.
> > > But moving MMIO/DMA/TLB etc.. barriers into this spinlock might not be a > > > good idea, since those are typically fairly heavy barriers, and its > > > quite common to call schedule() without ending up in switch_to(). > > > > That's true I guess, but if we already have the arch specific smp_mb__ > > specifically for this context switch code, and you are asking for them to > > implement *cacheable* memory barrier vs migration, then I see no reason > > not to allow them to implement uncacheable as well. > > > > You make a good point about schedule() without switch_to(), but > > architectures will still have no less flexibility than they do now. > > Ah, so you're saying make it optional where they put it? I was initially > thinking you wanted to add it to the list of requirements. Sure, > optional works.
Yes i.e., this primitive must provide minimally X, and optionally Y. If Y is not provided, then switch_to or other arch hook must provide it.
Thanks, Nick
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