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signal: align __lock_task_sighand() irq disabling and RCU

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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>
This commit is contained in:
Paul E. McKenney 2011-07-19 03:25:36 -07:00 committed by Paul E. McKenney
parent ec433f0c51
commit a841796f11
1 changed files with 13 additions and 6 deletions
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25
kernel/signal.c
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@ -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))
break;
spin_lock_irqsave(&sighand->siglock, *flags);
if (likely(sighand == tsk->sighand))
break;
spin_unlock_irqrestore(&sighand->siglock, *flags);
}
if (unlikely(sighand == NULL)) {
rcu_read_unlock();
local_irq_restore(*flags);
break;
}
spin_lock(&sighand->siglock);
if (likely(sighand == tsk->sighand)) {
rcu_read_unlock();
break;
}
spin_unlock(&sighand->siglock);
rcu_read_unlock();
local_irq_restore(*flags);
}
return sighand;
}