Paul,
I just noticed by accident that __lock_task_sighand() looks ugly and
mysterious ;) And I am puzzled.
a841796f11c90d53 "signal: align __lock_task_sighand() irq disabling and RCU"
says:
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 ...
OK, if we can't rcu_read_unlock() with irqs disabled, then we can at least
cleanup it (and document the problem). Say,
struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
unsigned long *flags)
{
struct sighand_struct *sighand;
rcu_read_lock();
for (;;) {
sighand = rcu_dereference(tsk->sighand);
if (unlikely(sighand == NULL))
break;
spin_lock_irqsave(&sighand->siglock, *flags);
/*
* We delay rcu_read_unlock() till unlock_task_sighand()
* to avoid rt_mutex_unlock(current->rcu_boost_mutex)
with
* irqs disabled.
*/
if (likely(sighand == tsk->sighand))
return sighand;
spin_unlock_irqrestore(&sighand->siglock, *flags);
}
rcu_read_unlock();
return sighand; /* NULL */
}
and add rcu_read_unlock() into unlock_task_sighand().
But. I simply can't understand why lockdep should complain? Why it is bad
to lock/unlock ->wait_lock with irqs disabled?
wakeup_next_waiter() and rt_mutex_adjust_prio() should be fine, they start
with _irqsave().
The changelog also says:
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.
and if it is actually bad, then how the change above can fix the problem?
Help!
Oleg.
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