bcachefs: Fix rcu_pending for PREEMPT_RT

PREEMPT_RT redefines how standard spinlocks work, so local_irq_save() +
spin_lock() is no longer equivalent to spin_lock_irqsave(). Fortunately,
we don't strictly need to do it that way.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/rcu_pending.c

@@ -182,11 +182,6 @@ static inline void kfree_bulk(size_t nr, void ** p)
 	while (nr--)
 		kfree(*p);
 }
-
-#define local_irq_save(flags)		\
-do {					\
-	flags = 0;			\
-} while (0)
 #endif
 
 static noinline void __process_finished_items(struct rcu_pending *pending,
@@ -429,9 +424,15 @@ __rcu_pending_enqueue(struct rcu_pending *pending, struct rcu_head *head,
 
 	BUG_ON((ptr != NULL) != (pending->process == RCU_PENDING_KVFREE_FN));
 
-	local_irq_save(flags);
-	p = this_cpu_ptr(pending->p);
-	spin_lock(&p->lock);
+	/* We could technically be scheduled before taking the lock and end up
+	 * using a different cpu's rcu_pending_pcpu: that's ok, it needs a lock
+	 * anyways
+	 *
+	 * And we have to do it this way to avoid breaking PREEMPT_RT, which
+	 * redefines how spinlocks work:
+	 */
+	p = raw_cpu_ptr(pending->p);
+	spin_lock_irqsave(&p->lock, flags);
 	rcu_gp_poll_state_t seq = __get_state_synchronize_rcu(pending->srcu);
 restart:
 	if (may_sleep &&
@@ -520,9 +521,8 @@ check_expired:
 		goto free_node;
 	}
 
-	local_irq_save(flags);
-	p = this_cpu_ptr(pending->p);
-	spin_lock(&p->lock);
+	p = raw_cpu_ptr(pending->p);
+	spin_lock_irqsave(&p->lock, flags);
 	goto restart;
 }