[PATCH tip/core/urgent 7/7] signal: align __lock_task_sighand() irq disabling and RCU

From: Paul E. McKenney
Date: Tue Jul 19 2011 - 20:19:16 EST

From: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>

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

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@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
kernel/signal.c | 16 +++++++++++-----
1 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index ff76786..a0eb019 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1178,18 +1178,24 @@ 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);
+ }

- spin_lock_irqsave(&sighand->siglock, *flags);
- if (likely(sighand == tsk->sighand))
+ spin_lock(&sighand->siglock);
+ if (likely(sighand == tsk->sighand)) {
+ rcu_read_unlock();
+ }
+ rcu_read_unlock();
spin_unlock_irqrestore(&sighand->siglock, *flags);
- rcu_read_unlock();

return sighand;

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