Re: [RFC][PATCH RT] rcu,rt: Allow rcu_read_lock_sched() to schedule

[Paul E. McKenney]

On Wed, Jun 26, 2013 at 09:42:13AM -0400, Steven Rostedt wrote:
> Looking at the 3.10 workqueue code, there's new code that adds
> local_irq_disable() around places that call spin locks which will turn
> into rt_mutexes for -rt. Reading the change log to why those
> local_irq_*() calls were added, it seems to just be to synchronize with
> synchronize_sched(). Talking with Tejun Heo, he may let me change those
> to rcu_read_lock_sched() as that is a more appropriate API for the
> purpose.
> 
> But that does not solve the -rt issue because even in -rt,
> rcu_read_lock_sched() disables preemption, which brings us to the
> purpose of this patch, to allow rcu_read_lock_sched() to preempt in -rt.
> 
> To allow rcu_read_lock_sched() sections to preempt in -rt, instead of
> disabling preemption, it will grab a local_lock(). Then the
> synchronize_sched() will grab all CPUs local_locks() and release them.
> After that, it still does the normal synchronize_sched() as there may be
> places that still disable preemption or irqs that it needs to
> synchronize with. By grabbing all the locks and releasing them, it will
> properly synchronize with those that use the locks instead of disabling
> preemption or interrupts.

I tried this approach in early 2005, and testing did not go well:

https://lkml.org/lkml/2005/3/17/199

For one thing, a lock acquired both within and surrounding an
RCU-sched read-side critical section would result in deadlock.

Or have things changed so that this now somehow works?

							Thanx, Paul

> Note: The rcu_read_lock_sched_notrace() version still only disables
> preemption, because they are used for lockdep and tracing, which require
> real preemption disabling and not mutexes.
> 
> Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> 
> Index: linux-rt.git/include/linux/rcupdate.h
> ===================================================================
> --- linux-rt.git.orig/include/linux/rcupdate.h
> +++ linux-rt.git/include/linux/rcupdate.h
> @@ -36,6 +36,7 @@
>  #include <linux/types.h>
>  #include <linux/cache.h>
>  #include <linux/spinlock.h>
> +#include <linux/locallock.h>
>  #include <linux/threads.h>
>  #include <linux/cpumask.h>
>  #include <linux/seqlock.h>
> @@ -870,6 +871,28 @@ static inline void rcu_read_unlock_bh(vo
>  	local_bh_enable();
>  }
> 
> +/* asm-offsets.c gets confused with local_lock here */
> +#if defined(CONFIG_PREEMPT_RT_FULL)
> +DECLARE_LOCAL_IRQ_LOCK(rcu_sched_lock);
> +static inline void rcu_read_lock_sched_disable(void)
> +{
> +	local_lock(rcu_sched_lock);
> +}
> +static inline void rcu_read_lock_sched_enable(void)
> +{
> +	local_unlock(rcu_sched_lock);
> +}
> +#else
> +static inline void rcu_read_lock_sched_disable(void)
> +{
> +	preempt_disable();
> +}
> +static inline void rcu_read_lock_sched_enable(void)
> +{
> +	preempt_enable();
> +}
> +#endif
> +
>  /**
>   * rcu_read_lock_sched() - mark the beginning of a RCU-sched critical section
>   *
> @@ -885,7 +908,7 @@ static inline void rcu_read_unlock_bh(vo
>   */
>  static inline void rcu_read_lock_sched(void)
>  {
> -	preempt_disable();
> +	rcu_read_lock_sched_disable();
>  	__acquire(RCU_SCHED);
>  	rcu_lock_acquire(&rcu_sched_lock_map);
>  	rcu_lockdep_assert(!rcu_is_cpu_idle(),
> @@ -910,7 +933,7 @@ static inline void rcu_read_unlock_sched
>  			   "rcu_read_unlock_sched() used illegally while idle");
>  	rcu_lock_release(&rcu_sched_lock_map);
>  	__release(RCU_SCHED);
> -	preempt_enable();
> +	rcu_read_lock_sched_enable();
>  }
> 
>  /* Used by lockdep and tracing: cannot be traced, cannot call lockdep. */
> Index: linux-rt.git/kernel/rcutree.c
> ===================================================================
> --- linux-rt.git.orig/kernel/rcutree.c
> +++ linux-rt.git/kernel/rcutree.c
> @@ -2491,6 +2491,31 @@ static inline int rcu_blocking_is_gp(voi
>  	return ret;
>  }
> 
> +#ifdef CONFIG_PREEMPT_RT_FULL
> +DEFINE_LOCAL_IRQ_LOCK(rcu_sched_lock);
> +/*
> + * Real-time allows for synchronize sched to sleep but not migrate.
> + * This is done via the local locks. When calling synchronize_sched(),
> + * all the local locks need to be taken and released. This will ensure
> + * that all users of rcu_read_lock_sched() will be out of their critical
> + * sections at the completion of this function. synchronize_sched() will
> + * still perform the normal RCU sched operations to synchronize with
> + * locations that use disabling preemption or interrupts.
> + */
> +static void rcu_synchronize_sched_rt(void)
> +{
> +	int cpu;
> +
> +	for_each_possible_cpu(cpu) {
> +		spin_lock(&per_cpu(rcu_sched_lock, cpu).lock);
> +		spin_unlock(&per_cpu(rcu_sched_lock, cpu).lock);
> +	}
> +}
> +#else
> +static inline void rcu_synchronize_sched_rt(void)
> +{
> +}
> +#endif
>  /**
>   * synchronize_sched - wait until an rcu-sched grace period has elapsed.
>   *
> @@ -2538,6 +2563,9 @@ void synchronize_sched(void)
>  			   !lock_is_held(&rcu_lock_map) &&
>  			   !lock_is_held(&rcu_sched_lock_map),
>  			   "Illegal synchronize_sched() in RCU-sched read-side critical section");
> +
> +	rcu_synchronize_sched_rt();
> +
>  	if (rcu_blocking_is_gp())
>  		return;
>  	if (rcu_expedited)
> 
> 

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