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    SubjectRe: [PATCH v2 26/31] arm64: Miscellaneous library functions
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    On Thursday 16 August 2012, Will Deacon wrote:
    > > > +
    > > > +#include <linux/kernel.h>
    > > > +#include <linux/spinlock.h>
    > > > +#include <linux/atomic.h>
    > > > +
    > > > +#ifdef CONFIG_SMP
    > > > +arch_spinlock_t __atomic_hash[ATOMIC_HASH_SIZE] __lock_aligned = {
    > > > + [0 ... (ATOMIC_HASH_SIZE-1)] = __ARCH_SPIN_LOCK_UNLOCKED
    > > > +};
    > > > +#endif
    > >
    > > What?
    > >
    > > I suppose this is a leftover from an earlier version using the
    > > generic bitops, right?
    >
    > We currently use the generic atomic bitops (asm-generic/bitops/atomic.h)
    > which contains:
    >
    > # define ATOMIC_HASH(a) (&(__atomic_hash[ (((unsigned long) a)/L1_CACHE_BYTES) & (ATOMIC_HASH_SIZE-1) ]))
    >
    > so we have to provide a definition for the array. We have additional patches
    > containing optimised assembly implementations of the atomic bitops which we
    > will push later, once we've got some hardware to benchmark with.
    >

    Ah, I was confusing this with the asm/atomic.h stuff, for which you already
    provide an optimized version.

    The generic atomic bitops are really horrible in performance and I would
    expect that there is just one obvious way to implement bitops using ldaxr/stlxr,
    so I recommend just doing that even if you have no hardware for benchmarking.

    The s390 version should be fairly easy to adapt.

    Arnd


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