[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.