futex: Optimize per-cpu reference counting

Shrikanth noted that the per-cpu reference counter was still some 10%
slower than the old immutable option (which removes the reference
counting entirely).

Further optimize the per-cpu reference counter by:

 - switching from RCU to preempt;
 - using __this_cpu_*() since we now have preempt disabled;
 - switching from smp_load_acquire() to READ_ONCE().

This is all safe because disabling preemption inhibits the RCU grace
period exactly like rcu_read_lock().

Having preemption disabled allows using __this_cpu_*() provided the
only access to the variable is in task context -- which is the case
here.

Furthermore, since we know changing fph->state to FR_ATOMIC demands a
full RCU grace period we can rely on the implied smp_mb() from that to
replace the acquire barrier().

This is very similar to the percpu_down_read_internal() fast-path.

The reason this is significant for PowerPC is that it uses the generic
this_cpu_*() implementation which relies on local_irq_disable() (the
x86 implementation relies on it being a single memop instruction to be
IRQ-safe). Switching to preempt_disable() and __this_cpu*() avoids
this IRQ state swizzling. Also, PowerPC needs LWSYNC for the ACQUIRE
barrier, not having to use explicit barriers safes a bunch.

Combined this reduces the performance gap by half, down to some 5%.

Fixes: 760e6f7 ("futex: Remove support for IMMUTABLE")
Reported-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Tested-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20251106092929.GR4067720@noisy.programming.kicks-ass.net

Author: Peter Zijlstra

kernel/futex/core.c

@@ -1680,10 +1680,10 @@ static bool futex_ref_get(struct futex_private_hash *fph)
 {
 	struct mm_struct *mm = fph->mm;
 
-	guard(rcu)();
+	guard(preempt)();
 
-	if (smp_load_acquire(&fph->state) == FR_PERCPU) {
-		this_cpu_inc(*mm->futex_ref);
+	if (READ_ONCE(fph->state) == FR_PERCPU) {
+		__this_cpu_inc(*mm->futex_ref);
 		return true;
 	}
 
@@ -1694,10 +1694,10 @@ static bool futex_ref_put(struct futex_private_hash *fph)
 {
 	struct mm_struct *mm = fph->mm;
 
-	guard(rcu)();
+	guard(preempt)();
 
-	if (smp_load_acquire(&fph->state) == FR_PERCPU) {
-		this_cpu_dec(*mm->futex_ref);
+	if (READ_ONCE(fph->state) == FR_PERCPU) {
+		__this_cpu_dec(*mm->futex_ref);
 		return false;
 	}