rcu-tasks: Remove open-coded one-byte cmpxchg() emulation

This commit removes the open-coded one-byte cmpxchg() emulation from
rcu_trc_cmpxchg_need_qs(), replacing it with just cmpxchg() given the
latter's new-found ability to handle single-byte arguments across all
architectures.

Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Neeraj Upadhyay <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>

Author: paulmckrcu

kernel/rcu/tasks.h

@@ -1541,22 +1541,7 @@ static void rcu_st_need_qs(struct task_struct *t, u8 v)
  */
 u8 rcu_trc_cmpxchg_need_qs(struct task_struct *t, u8 old, u8 new)
 {
-	union rcu_special ret;
-	union rcu_special trs_old = READ_ONCE(t->trc_reader_special);
-	union rcu_special trs_new = trs_old;
-
-	if (trs_old.b.need_qs != old)
-		return trs_old.b.need_qs;
-	trs_new.b.need_qs = new;
-
-	// Although cmpxchg() appears to KCSAN to update all four bytes,
-	// only the .b.need_qs byte actually changes.
-	instrument_atomic_read_write(&t->trc_reader_special.b.need_qs,
-				     sizeof(t->trc_reader_special.b.need_qs));
-	// Avoid false-positive KCSAN failures.
-	ret.s = data_race(cmpxchg(&t->trc_reader_special.s, trs_old.s, trs_new.s));
-
-	return ret.b.need_qs;
+	return cmpxchg(&t->trc_reader_special.b.need_qs, old, new);
 }
 EXPORT_SYMBOL_GPL(rcu_trc_cmpxchg_need_qs);