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SubjectRe: [RFC PATCH 3/5] asm-generic/bitops/atomic.h: Rewrite using atomic_fetch_*
On Thu, Feb 15, 2018 at 03:29:33PM +0000, Will Deacon wrote:
> +static inline void set_bit(unsigned int nr, volatile unsigned long *p)
> +{
> + p += BIT_WORD(nr);
> + atomic_long_fetch_or_relaxed(BIT_MASK(nr), (atomic_long_t *)p);
> +}
>
> +static inline void clear_bit(unsigned int nr, volatile unsigned long *p)
> +{
> + p += BIT_WORD(nr);
> + atomic_long_fetch_andnot_relaxed(BIT_MASK(nr), (atomic_long_t *)p);
> +}
>
> +static inline void change_bit(unsigned int nr, volatile unsigned long *p)
> +{
> + p += BIT_WORD(nr);
> + atomic_long_fetch_xor_relaxed(BIT_MASK(nr), (atomic_long_t *)p);
> +}
>
> +static inline int test_and_set_bit(unsigned int nr, volatile unsigned long *p)
> {
> + long old;
> unsigned long mask = BIT_MASK(nr);
>
> + p += BIT_WORD(nr);
> + if (READ_ONCE(*p) & mask)
> + return 1;
> +
> + old = atomic_long_fetch_or(mask, (atomic_long_t *)p);
> + return !!(old & mask);
> }
>
> +static inline int test_and_clear_bit(unsigned int nr, volatile unsigned long *p)
> {
> + long old;
> unsigned long mask = BIT_MASK(nr);
>
> + p += BIT_WORD(nr);
> + if (!(READ_ONCE(*p) & mask))
> + return 0;
> +
> + old = atomic_long_fetch_andnot(mask, (atomic_long_t *)p);
> + return !!(old & mask);
> }
>
> +static inline int test_and_change_bit(unsigned int nr, volatile unsigned long *p)
> {
> + long old;
> unsigned long mask = BIT_MASK(nr);
>
> + p += BIT_WORD(nr);
> + old = atomic_long_fetch_xor(mask, (atomic_long_t *)p);
> + return !!(old & mask);
> }
>
> +static inline int test_and_set_bit_lock(unsigned int nr,
> + volatile unsigned long *p)
> {
> + long old;
> unsigned long mask = BIT_MASK(nr);
>
> + p += BIT_WORD(nr);
> + if (READ_ONCE(*p) & mask)
> + return 1;
>
> + old = atomic_long_fetch_or_acquire(mask, (atomic_long_t *)p);
> + return !!(old & mask);
> }
>
> +static inline void clear_bit_unlock(unsigned int nr, volatile unsigned long *p)
> {
> + p += BIT_WORD(nr);
> + atomic_long_fetch_andnot_release(BIT_MASK(nr), (atomic_long_t *)p);
> +}
>
> +static inline void __clear_bit_unlock(unsigned int nr,
> + volatile unsigned long *p)
> +{
> + unsigned long old;
>
> + p += BIT_WORD(nr);
> + old = READ_ONCE(*p);
> + old &= ~BIT_MASK(nr);
> + smp_store_release(p, old);

This should be atomic_set_release() I think, for the special case where
atomics are implemented with spinlocks, see the 'fun' case in
Documentation/atomic_t.txt.

> }

The only other comment is that I think it would be better if you use
atomic_t instead of atomic_long_t. It would just mean changing
BIT_WORD() and BIT_MASK().

The reason is that we generate a pretty sane set of atomic_t primitives
as long as the architecture supplies cmpxchg, but atomic64 defaults to
utter crap, even on 64bit platforms.

Otherwise this looks pretty neat.

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