locking/pvqspinlock/x86: Remove redundant CMP after CMPXCHG in...

locking/pvqspinlock/x86: Remove redundant CMP after CMPXCHG in __raw_callee_save___pv_queued_spin_unlock()

x86 CMPXCHG instruction returns success in the ZF flag. Remove
redundant CMP instruction after CMPXCHG that performs the same check.

Also update the function comment to mention the modern version
of the equivalent C code.
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240412083908.282802-1-ubizjak@gmail.com

arch/x86/include/asm/qspinlock_paravirt.h

@@ -25,9 +25,9 @@ __PV_CALLEE_SAVE_REGS_THUNK(__pv_queued_spin_unlock_slowpath, ".spinlock.text");
  *
  * void __lockfunc __pv_queued_spin_unlock(struct qspinlock *lock)
  * {
- *	u8 lockval = cmpxchg(&lock->locked, _Q_LOCKED_VAL, 0);
+ *	u8 lockval = _Q_LOCKED_VAL;
  *
- *	if (likely(lockval == _Q_LOCKED_VAL))
+ *	if (try_cmpxchg(&lock->locked, &lockval, 0))
  *		return;
  *	pv_queued_spin_unlock_slowpath(lock, lockval);
  * }
@@ -43,7 +43,6 @@ __PV_CALLEE_SAVE_REGS_THUNK(__pv_queued_spin_unlock_slowpath, ".spinlock.text");
 	"mov   $0x1,%eax\n\t"						\
 	"xor   %edx,%edx\n\t"						\
 	LOCK_PREFIX "cmpxchg %dl,(%rdi)\n\t"				\
-	"cmp   $0x1,%al\n\t"						\
 	"jne   .slowpath\n\t"						\
 	"pop   %rdx\n\t"						\
 	FRAME_END							\