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    SubjectRe: [RFC][PATCH] mips: Fix arch_spin_unlock()
    On Tue, Feb 02, 2016 at 10:06:36AM -0800, Linus Torvalds wrote:
    > On Tue, Feb 2, 2016 at 9:51 AM, Will Deacon <will.deacon@arm.com> wrote:
    > >
    > > Given that the vast majority of weakly ordered architectures respect
    > > address dependencies, I would expect all of them to be hurt if they
    > > were forced to use barrier instructions instead, even those where the
    > > microarchitecture is fairly strongly ordered in practice.
    >
    > I do wonder if it would be all that noticeable, though. I don't think
    > we've really had benchmarks.
    >
    > For example, most of the RCU list traversal shows up on x86 - where
    > loads are already acquires. But they show up not because of that, but
    > because a RCU list traversal is pretty much always going to take the
    > cache miss.
    >
    > So it would actually be interesting to just try it - what happens to
    > kernel-centric benchmarks (which are already fairly rare) on arm if we
    > change the rcu_dereference() to be a smp_load_acquire()?
    >
    > Because maybe nothing happens at all. I don't think we've ever tried it.

    FWIW, and this is by no means conclusive, I hacked that up quickly and
    ran hackbench a few times on the nearest idle arm64 system. The results
    were consistently ~4% slower using acquire for rcu_dereference.

    Will

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