[PATCH 1/2] signal: simplify deadlock-avoidance in lock_task_sighand()

[Oleg Nesterov <oleg@redhat.com>]

__lock_task_sighand() does local_irq_save() to prevent the potential
deadlock, we can use preempt_disable() with the same effect. And in
this case we can do preempt_disable/enable + rcu_read_lock/unlock only
once outside of the main loop and simplify the code. This also shaves
112 bytes from signal.o.

With this patch the main loop runs with preemption disabled, but this
should be fine because restart is very unlikely: it can only happen if
we race with de_thread() and ->sighand is shared. And the latter is only
possible if CLONE_SIGHAND was used without CLONE_THREAD, most probably
nobody does this nowadays.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/signal.c |   31 +++++++++++++------------------
 1 files changed, 13 insertions(+), 18 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 8f0876f..61a1f55 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1261,30 +1261,25 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
 					   unsigned long *flags)
 {
 	struct sighand_struct *sighand;
-
+	/*
+	 * We are going to do rcu_read_unlock() under spin_lock_irqsave().
+	 * Make sure we can not be preempted after rcu_read_lock(), see
+	 * rcu_read_unlock() comment header for details.
+	 */
+	preempt_disable();
+	rcu_read_lock();
 	for (;;) {
-		/*
-		 * Disable interrupts early to avoid deadlocks.
-		 * See rcu_read_unlock() comment header for details.
-		 */
-		local_irq_save(*flags);
-		rcu_read_lock();
 		sighand = rcu_dereference(tsk->sighand);
-		if (unlikely(sighand == NULL)) {
-			rcu_read_unlock();
-			local_irq_restore(*flags);
+		if (unlikely(sighand == NULL))
 			break;
-		}
 
-		spin_lock(&sighand->siglock);
-		if (likely(sighand == tsk->sighand)) {
-			rcu_read_unlock();
+		spin_lock_irqsave(&sighand->siglock, *flags);
+		if (likely(sighand == tsk->sighand))
 			break;
-		}
-		spin_unlock(&sighand->siglock);
-		rcu_read_unlock();
-		local_irq_restore(*flags);
+		spin_unlock_irqrestore(&sighand->siglock, *flags);
 	}
+	rcu_read_unlock();
+	preempt_enable();
 
 	return sighand;
 }
-- 
1.5.5.1