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>

1 files changed, 13 insertions, 6 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index ff7678603328..415d85d6f6c6 100644
--- a/kernel/signal.c
+++ b/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;
 }