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__clear_bit_unlock() is a special little snowflake. While it carries the
non-atomic '__' prefix, it is specifically documented to pair with
test_and_set_bit() and therefore should be 'somewhat' atomic.
Therefore the generic implementation of __clear_bit_unlock() cannot use
the fully non-atomic __clear_bit() as a default.
If an arch is able to do better; is must provide an implementation of
__clear_bit_unlock() itself.
Specifically, this came up as a result of hackbench livelock'ing in
slab_lock() on ARC with SMP + SLUB + !LLSC.
The issue was incorrect pairing of atomic ops.
slab_lock() -> bit_spin_lock() -> test_and_set_bit()
slab_unlock() -> __bit_spin_unlock() -> __clear_bit()
The non serializing __clear_bit() was getting "lost"
80543b8e: ld_s r2,[r13,0] <--- (A) Finds PG_locked is set
80543b90: or r3,r2,1 <--- (B) other core unlocks right here
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| .. | ||
| __ffs.h | ||
| __fls.h | ||
| arch_hweight.h | ||
| atomic.h | ||
| builtin-__ffs.h | ||
| builtin-__fls.h | ||
| builtin-ffs.h | ||
| builtin-fls.h | ||
| const_hweight.h | ||
| ext2-atomic-setbit.h | ||
| ext2-atomic.h | ||
| ffs.h | ||
| ffz.h | ||
| find.h | ||
| fls64.h | ||
| fls.h | ||
| hweight.h | ||
| le.h | ||
| lock.h | ||
| non-atomic.h | ||
| sched.h | ||