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SubjectRe: [PATCH tip/core/rcu 41/55] rcu: Permit rt_mutex_unlock() with irqs disabled
On Tue, Sep 06, 2011 at 11:00:35AM -0700, Paul E. McKenney wrote:
> From: Paul E. McKenney <paul.mckenney@linaro.org>
>
> Create a separate lockdep class for the rt_mutex used for RCU priority
> boosting and enable use of rt_mutex_lock() with irqs disabled. This
> prevents RCU priority boosting from falling prey to deadlocks when
> someone begins an RCU read-side critical section in preemptible state,
> but releases it with an irq-disabled lock held.
>
> Unfortunately, the scheduler's runqueue and priority-inheritance locks
> still must either completely enclose or be completely enclosed by any
> overlapping RCU read-side critical section.
>
> Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> kernel/rcutree_plugin.h | 6 ++++++
> kernel/rtmutex.c | 8 ++++++++
> 2 files changed, 14 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index d3127e8..f6c63ea 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -1149,6 +1149,8 @@ static void rcu_initiate_boost_trace(struct rcu_node *rnp)
>
> #endif /* #else #ifdef CONFIG_RCU_TRACE */
>
> +static struct lock_class_key rcu_boost_class;
> +
> /*
> * Carry out RCU priority boosting on the task indicated by ->exp_tasks
> * or ->boost_tasks, advancing the pointer to the next task in the
> @@ -1211,10 +1213,14 @@ static int rcu_boost(struct rcu_node *rnp)
> */
> t = container_of(tb, struct task_struct, rcu_node_entry);
> rt_mutex_init_proxy_locked(&mtx, t);
> + /* Avoid lockdep false positives. This rt_mutex is its own thing. */
> + lockdep_set_class_and_name(&mtx.wait_lock, &rcu_boost_class,
> + "rcu_boost_mutex");
> t->rcu_boost_mutex = &mtx;

raw_spin_unlock_irqrestore(&rnp->lock, flags); <====A
> rt_mutex_lock(&mtx); /* Side effect: boosts task t's priority. */
> rt_mutex_unlock(&mtx); /* Keep lockdep happy. */
> + local_irq_restore(flags);

Does it help here?
irq is enabled at A. So we still call rt_mutex_lock() with irq enabled.

Seems should s/raw_spin_unlock_irqrestore/raw_spin_unlock ?

BTW, since we are in process context, 'flags' is not needed to save,
no?

Thanks,
Yong


>
> return rnp->exp_tasks != NULL || rnp->boost_tasks != NULL;
> }
> diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
> index ab44911..2548f44 100644
> --- a/kernel/rtmutex.c
> +++ b/kernel/rtmutex.c
> @@ -579,6 +579,7 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
> struct rt_mutex_waiter *waiter)
> {
> int ret = 0;
> + int was_disabled;
>
> for (;;) {
> /* Try to acquire the lock: */
> @@ -601,10 +602,17 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
>
> raw_spin_unlock(&lock->wait_lock);
>
> + was_disabled = irqs_disabled();
> + if (was_disabled)
> + local_irq_enable();
> +
> debug_rt_mutex_print_deadlock(waiter);
>
> schedule_rt_mutex(lock);
>
> + if (was_disabled)
> + local_irq_disable();
> +
> raw_spin_lock(&lock->wait_lock);
> set_current_state(state);
> }
> --
> 1.7.3.2
>
> --
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