locking/qspinlock: Use atomic_try_cmpxchg_relaxed() in xchg_tail()

locking/qspinlock: Use atomic_try_cmpxchg_relaxed() in xchg_tail()

Use atomic_try_cmpxchg_relaxed(*ptr, &old, new) instead of
atomic_cmpxchg_relaxed (*ptr, old, new) == old in xchg_tail().

x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after CMPXCHG.

No functional change intended.

Since this code requires NR_CPUS >= 16k, I have tested it
by unconditionally setting _Q_PENDING_BITS to 1 in
<asm-generic/qspinlock_types.h>.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240321195309.484275-1-ubizjak@gmail.com