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    SubjectRe: [PATCH v2 26/31] arm64: Miscellaneous library functions
    On Thu, Aug 16, 2012 at 02:00:32PM +0100, Arnd Bergmann wrote:
    > 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.

    As Will said, we have the code already but I dropped it from the initial
    set patches to be reviewed to keep them simpler. They will be added
    later.

    --
    Catalin


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