signal: align __lock_task_sighand() irq disabling and RCU

The __lock_task_sighand() function calls rcu_read_lock() with interrupts
and preemption enabled, but later calls rcu_read_unlock() with interrupts
disabled.  It is therefore possible that this RCU read-side critical
section will be preempted and later RCU priority boosted, which means that
rcu_read_unlock() will call rt_mutex_unlock() in order to deboost itself, but
with interrupts disabled. This results in lockdep splats, so this commit
nests the RCU read-side critical section within the interrupt-disabled
region of code.  This prevents the RCU read-side critical section from
being preempted, and thus prevents the attempt to deboost with interrupts
disabled.

It is quite possible that a better long-term fix is to make rt_mutex_unlock()
disable irqs when acquiring the rt_mutex structure's ->wait_lock.
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/signal.c

@@ -1178,18 +1178,25 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
 {
 	struct sighand_struct *sighand;
 
-	rcu_read_lock();
 	for (;;) {
+		local_irq_save(*flags);
+		rcu_read_lock();
 		sighand = rcu_dereference(tsk->sighand);
-		if (unlikely(sighand == NULL))
+		if (unlikely(sighand == NULL)) {
+			rcu_read_unlock();
+			local_irq_restore(*flags);
 			break;
+		}
 
-		spin_lock_irqsave(&sighand->siglock, *flags);
-		if (likely(sighand == tsk->sighand))
+		spin_lock(&sighand->siglock);
+		if (likely(sighand == tsk->sighand)) {
+			rcu_read_unlock();
 			break;
-		spin_unlock_irqrestore(&sighand->siglock, *flags);
+		}
+		spin_unlock(&sighand->siglock);
+		rcu_read_unlock();
+		local_irq_restore(*flags);
 	}
-	rcu_read_unlock();
 
 	return sighand;
 }