Commit a841796f authored Jul 19, 2011 by Paul E. McKenney Committed by Paul E. McKenney Jul 20, 2011

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>

parent ec433f0c

kernel/signal.c

+13 −6

Original line number	Diff line number	Diff line
		@@ -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);
		}

		return sighand;
		}