Re: [PATCH 1/7] rcu: Create rcu_sync infrastructure

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, Jul 12, 2015 at 01:35:48AM +0200, Oleg Nesterov wrote:
> It is functionally equivalent to
> 
>         struct rcu_sync_struct {
>                 atomic_t counter;
>         };
> 
>         static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
>         {

If you add an smp_mb() here...

>                 return atomic_read(&rss->counter) == 0;
>         }
> 
>         static inline void rcu_sync_enter(struct rcu_sync_struct *rss)
>         {
>                 atomic_inc(&rss->counter);
>                 synchronize_sched();
>         }
> 
>         static inline void rcu_sync_exit(struct rcu_sync_struct *rss)
>         {
>                 synchronize_sched();

You should be able to demote the above synchronize_sched() to an
smp_mb__before_atomic().  Even rare writes should make this tradeoff
worthwhile.

I will try to call out the equivalent transformation below, for your
amusement if for no other reason.  ;-)

>                 atomic_dec(&rss->counter);
>         }
> 
> except: it records the state and synchronize_sched() is only called by
> rcu_sync_enter() and only if necessary.
> 
> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  include/linux/rcusync.h |   64 ++++++++++++++++++++++++++++
>  kernel/rcu/Makefile     |    2 +-
>  kernel/rcu/sync.c       |  108 +++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 173 insertions(+), 1 deletions(-)
>  create mode 100644 include/linux/rcusync.h
>  create mode 100644 kernel/rcu/sync.c
> 
> diff --git a/include/linux/rcusync.h b/include/linux/rcusync.h
> new file mode 100644
> index 0000000..7858491
> --- /dev/null
> +++ b/include/linux/rcusync.h
> @@ -0,0 +1,64 @@
> +#ifndef _LINUX_RCUSYNC_H_
> +#define _LINUX_RCUSYNC_H_
> +
> +#include <linux/wait.h>
> +#include <linux/rcupdate.h>
> +
> +struct rcu_sync_struct {
> +	int			gp_state;
> +	int			gp_count;
> +	wait_queue_head_t	gp_wait;
> +
> +	int			cb_state;
> +	struct rcu_head		cb_head;
> +
> +	void (*sync)(void);
> +	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
> +};
> +
> +#define ___RCU_SYNC_INIT(name)						\
> +	.gp_state = 0,							\
> +	.gp_count = 0,							\
> +	.gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait),		\
> +	.cb_state = 0
> +
> +#define __RCU_SCHED_SYNC_INIT(name) {					\
> +	___RCU_SYNC_INIT(name),						\
> +	.sync = synchronize_sched,					\
> +	.call = call_rcu_sched,						\
> +}
> +
> +#define __RCU_BH_SYNC_INIT(name) {					\
> +	___RCU_SYNC_INIT(name),						\
> +	.sync = synchronize_rcu_bh,					\
> +	.call = call_rcu_bh,						\
> +}
> +
> +#define __RCU_SYNC_INIT(name) {						\
> +	___RCU_SYNC_INIT(name),						\
> +	.sync = synchronize_rcu,					\
> +	.call = call_rcu,						\
> +}
> +
> +#define DEFINE_RCU_SCHED_SYNC(name)					\
> +	struct rcu_sync_struct name = __RCU_SCHED_SYNC_INIT(name)
> +
> +#define DEFINE_RCU_BH_SYNC(name)					\
> +	struct rcu_sync_struct name = __RCU_BH_SYNC_INIT(name)
> +
> +#define DEFINE_RCU_SYNC(name)						\
> +	struct rcu_sync_struct name = __RCU_SYNC_INIT(name)
> +
> +static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
> +{

	smp_mb();  /* A: Ensure that reader sees last update. */
		   /* Pairs with B. */

> +	return !rss->gp_state; /* GP_IDLE */
> +}
> +
> +enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
> +
> +extern void rcu_sync_init(struct rcu_sync_struct *, enum rcu_sync_type);
> +extern void rcu_sync_enter(struct rcu_sync_struct *);
> +extern void rcu_sync_exit(struct rcu_sync_struct *);
> +
> +#endif /* _LINUX_RCUSYNC_H_ */
> +
> diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile
> index 50a8084..61a1656 100644
> --- a/kernel/rcu/Makefile
> +++ b/kernel/rcu/Makefile
> @@ -1,4 +1,4 @@
> -obj-y += update.o
> +obj-y += update.o sync.o
>  obj-$(CONFIG_SRCU) += srcu.o
>  obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
>  obj-$(CONFIG_TREE_RCU) += tree.o
> diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> new file mode 100644
> index 0000000..f84176a
> --- /dev/null
> +++ b/kernel/rcu/sync.c
> @@ -0,0 +1,108 @@
> +
> +#include <linux/rcusync.h>
> +#include <linux/sched.h>
> +
> +enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
> +enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
> +
> +#define	rss_lock	gp_wait.lock
> +
> +void rcu_sync_init(struct rcu_sync_struct *rss, enum rcu_sync_type type)
> +{
> +	memset(rss, 0, sizeof(*rss));
> +	init_waitqueue_head(&rss->gp_wait);
> +
> +	switch (type) {
> +	case RCU_SYNC:
> +		rss->sync = synchronize_rcu;
> +		rss->call = call_rcu;
> +		break;
> +
> +	case RCU_SCHED_SYNC:
> +		rss->sync = synchronize_sched;
> +		rss->call = call_rcu_sched;
> +		break;
> +
> +	case RCU_BH_SYNC:
> +		rss->sync = synchronize_rcu_bh;
> +		rss->call = call_rcu_bh;
> +		break;
> +	}
> +}
> +
> +void rcu_sync_enter(struct rcu_sync_struct *rss)
> +{
> +	bool need_wait, need_sync;
> +
> +	spin_lock_irq(&rss->rss_lock);
> +	need_wait = rss->gp_count++;
> +	need_sync = rss->gp_state == GP_IDLE;
> +	if (need_sync)
> +		rss->gp_state = GP_PENDING;
> +	spin_unlock_irq(&rss->rss_lock);
> +
> +	BUG_ON(need_wait && need_sync);
> +
> +	if (need_sync) {
> +		rss->sync();
> +		rss->gp_state = GP_PASSED;
> +		wake_up_all(&rss->gp_wait);
> +	} else if (need_wait) {
> +		wait_event(rss->gp_wait, rss->gp_state == GP_PASSED);
> +	} else {
> +		/*
> +		 * Possible when there's a pending CB from a rcu_sync_exit().
> +		 * Nobody has yet been allowed the 'fast' path and thus we can
> +		 * avoid doing any sync(). The callback will get 'dropped'.
> +		 */
> +		BUG_ON(rss->gp_state != GP_PASSED);
> +	}
> +}
> +
> +static void rcu_sync_func(struct rcu_head *rcu)
> +{
> +	struct rcu_sync_struct *rss =
> +		container_of(rcu, struct rcu_sync_struct, cb_head);
> +	unsigned long flags;
> +
> +
> +	BUG_ON(rss->gp_state != GP_PASSED);
> +	BUG_ON(rss->cb_state == CB_IDLE);
> +
> +	spin_lock_irqsave(&rss->rss_lock, flags);
> +	if (rss->gp_count) {
> +		/*
> +		 * A new rcu_sync_begin() has happened; drop the callback.
> +		 */
> +		rss->cb_state = CB_IDLE;
> +	} else if (rss->cb_state == CB_REPLAY) {
> +		/*
> +		 * A new rcu_sync_exit() has happened; requeue the callback
> +		 * to catch a later GP.
> +		 */
> +		rss->cb_state = CB_PENDING;
> +		rss->call(&rss->cb_head, rcu_sync_func);
> +	} else {
> +		/*
> +		 * We're at least a GP after rcu_sync_exit(); eveybody will now
> +		 * have observed the write side critical section. Let 'em rip!.
> +		 */
> +		rss->cb_state = CB_IDLE;
> +		rss->gp_state = GP_IDLE;
> +	}
> +	spin_unlock_irqrestore(&rss->rss_lock, flags);
> +}
> +
> +void rcu_sync_exit(struct rcu_sync_struct *rss)
> +{
> +	spin_lock_irq(&rss->rss_lock);

	smp_mb(); /* B: Make sure next readers see critical section. */
		  /* Pairs with A. */

> +	if (!--rss->gp_count) {

At which point, I believe you can ditch the callback entirely, along
with ->cb_state.

So, what am I missing here?  Are readers really so frequent that the
added read-side memory barrier is visible?  Given that the current
code forces the readers to grab ->rss_lock during the trailing grace
period, any writer frequency at all will overwhelm the cost of the
read-side memory barrier.

							Thanx, Paul

> +		if (rss->cb_state == CB_IDLE) {
> +			rss->cb_state = CB_PENDING;
> +			rss->call(&rss->cb_head, rcu_sync_func);
> +		} else if (rss->cb_state == CB_PENDING) {
> +			rss->cb_state = CB_REPLAY;
> +		}
> +	}
> +	spin_unlock_irq(&rss->rss_lock);
> +}
> -- 
> 1.5.5.1
>