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    SubjectRe: Question on smp_mb__before_spinlock
    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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