Re: [PATCH 1/1] rcu/sync: simplify the state machine

[Paul E. McKenney]

On Thu, Apr 25, 2019 at 06:50:55PM +0200, Oleg Nesterov wrote:
> With this patch rcu_sync has a single state variable and the transition rules
> become really simple:
> 
> 	GP_IDLE   - owned by the first rcu_sync_enter() which moves it to
> 
> 	GP_ENTER  - owned by rcu-callback which moves it to
> 
> 	GP_PASSED - owned by the last rcu_sync_exit() which moves it to
> 
> 	GP_EXIT   - and this is the only "nontrivial" state.
> 
> 		rcu-callback moves it back to GP_IDLE unless another enter()
> 		comes before a GP pass.
> 
> 		If rcu-callback is invoked before the next rcu_sync_exit() it
> 		must see gp_count incremented by that enter() and set GP_PASSED.
> 
> 		Otherwise, if the next rcu_sync_exit() wins the race, it will
> 		move it to
> 
> 	GP_REPLAY - owned by rcu-callback which moves it to GP_EXIT
> 
> Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>

Queued and passing initial tests, thank you!  It may be a day or two
before it shows up on -rcu, but it will get there!

							Thanx, Paul

> ---
>  include/linux/rcu_sync.h |   2 -
>  kernel/rcu/sync.c        | 165 +++++++++++++++++++++++++++--------------------
>  2 files changed, 95 insertions(+), 72 deletions(-)
> 
> diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
> index e7ae221..3156a14 100644
> --- a/include/linux/rcu_sync.h
> +++ b/include/linux/rcu_sync.h
> @@ -19,7 +19,6 @@ struct rcu_sync {
>  	int			gp_count;
>  	wait_queue_head_t	gp_wait;
>  
> -	int			cb_state;
>  	struct rcu_head		cb_head;
>  };
>  
> @@ -47,7 +46,6 @@ extern void rcu_sync_dtor(struct rcu_sync *);
>  		.gp_state = 0,						\
>  		.gp_count = 0,						\
>  		.gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait),	\
> -		.cb_state = 0,						\
>  	}
>  
>  #define	DEFINE_RCU_SYNC(name)	\
> diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
> index ee427e1..d9f80fc 100644
> --- a/kernel/rcu/sync.c
> +++ b/kernel/rcu/sync.c
> @@ -10,15 +10,13 @@
>  #include <linux/rcu_sync.h>
>  #include <linux/sched.h>
>  
> -enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
> -enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
> +enum { GP_IDLE = 0, GP_ENTER, GP_PASSED, GP_EXIT, GP_REPLAY };
>  
>  #define	rss_lock	gp_wait.lock
>  
>  /**
>   * rcu_sync_init() - Initialize an rcu_sync structure
>   * @rsp: Pointer to rcu_sync structure to be initialized
> - * @type: Flavor of RCU with which to synchronize rcu_sync structure
>   */
>  void rcu_sync_init(struct rcu_sync *rsp)
>  {
> @@ -41,56 +39,26 @@ void rcu_sync_enter_start(struct rcu_sync *rsp)
>  	rsp->gp_state = GP_PASSED;
>  }
>  
> -/**
> - * rcu_sync_enter() - Force readers onto slowpath
> - * @rsp: Pointer to rcu_sync structure to use for synchronization
> - *
> - * This function is used by updaters who need readers to make use of
> - * a slowpath during the update.  After this function returns, all
> - * subsequent calls to rcu_sync_is_idle() will return false, which
> - * tells readers to stay off their fastpaths.  A later call to
> - * rcu_sync_exit() re-enables reader slowpaths.
> - *
> - * When called in isolation, rcu_sync_enter() must wait for a grace
> - * period, however, closely spaced calls to rcu_sync_enter() can
> - * optimize away the grace-period wait via a state machine implemented
> - * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
> - */
> -void rcu_sync_enter(struct rcu_sync *rsp)
> -{
> -	bool need_wait, need_sync;
>  
> -	spin_lock_irq(&rsp->rss_lock);
> -	need_wait = rsp->gp_count++;
> -	need_sync = rsp->gp_state == GP_IDLE;
> -	if (need_sync)
> -		rsp->gp_state = GP_PENDING;
> -	spin_unlock_irq(&rsp->rss_lock);
> +static void rcu_sync_func(struct rcu_head *rcu);
>  
> -	WARN_ON_ONCE(need_wait && need_sync);
> -	if (need_sync) {
> -		synchronize_rcu();
> -		rsp->gp_state = GP_PASSED;
> -		wake_up_all(&rsp->gp_wait);
> -	} else if (need_wait) {
> -		wait_event(rsp->gp_wait, rsp->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'.
> -		 */
> -		WARN_ON_ONCE(rsp->gp_state != GP_PASSED);
> -	}
> +static void rcu_sync_call(struct rcu_sync *rsp)
> +{
> +	call_rcu(&rsp->cb_head, rcu_sync_func);
>  }
>  
>  /**
>   * rcu_sync_func() - Callback function managing reader access to fastpath
>   * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
>   *
> - * This function is passed to one of the call_rcu() functions by
> + * This function is passed to call_rcu() function by rcu_sync_enter() and
>   * rcu_sync_exit(), so that it is invoked after a grace period following the
> - * that invocation of rcu_sync_exit().  It takes action based on events that
> + * that invocation of enter/exit.
> + *
> + * If it is called by rcu_sync_enter() it signals that all the readers were
> + * switched onto slow path.
> + *
> + * If it is called by rcu_sync_exit() it takes action based on events that
>   * have taken place in the meantime, so that closely spaced rcu_sync_enter()
>   * and rcu_sync_exit() pairs need not wait for a grace period.
>   *
> @@ -102,40 +70,93 @@ void rcu_sync_enter(struct rcu_sync *rsp)
>   * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
>   * can again use their fastpaths.
>   */
> -static void rcu_sync_func(struct rcu_head *rhp)
> +static void rcu_sync_func(struct rcu_head *rcu)
>  {
> -	struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
> +	struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head);
>  	unsigned long flags;
>  
> -	WARN_ON_ONCE(rsp->gp_state != GP_PASSED);
> -	WARN_ON_ONCE(rsp->cb_state == CB_IDLE);
> +	WARN_ON_ONCE(rsp->gp_state == GP_IDLE);
> +	WARN_ON_ONCE(rsp->gp_state == GP_PASSED);
>  
>  	spin_lock_irqsave(&rsp->rss_lock, flags);
>  	if (rsp->gp_count) {
>  		/*
> -		 * A new rcu_sync_begin() has happened; drop the callback.
> +		 * We're at least a GP after the GP_IDLE->GP_ENTER transition.
>  		 */
> -		rsp->cb_state = CB_IDLE;
> -	} else if (rsp->cb_state == CB_REPLAY) {
> +		rsp->gp_state = GP_PASSED;
> +		wake_up_locked(&rsp->gp_wait);
> +	} else if (rsp->gp_state == GP_REPLAY) {
>  		/*
> -		 * A new rcu_sync_exit() has happened; requeue the callback
> -		 * to catch a later GP.
> +		 * A new rcu_sync_exit() has happened; requeue the callback to
> +		 * catch a later GP.
>  		 */
> -		rsp->cb_state = CB_PENDING;
> -		call_rcu(&rsp->cb_head, rcu_sync_func);
> +		rsp->gp_state = GP_EXIT;
> +		rcu_sync_call(rsp);
>  	} else {
>  		/*
> -		 * We're at least a GP after rcu_sync_exit(); eveybody will now
> -		 * have observed the write side critical section. Let 'em rip!.
> +		 * We're at least a GP after the last rcu_sync_exit(); eveybody
> +		 * will now have observed the write side critical section.
> +		 * Let 'em rip!.
>  		 */
> -		rsp->cb_state = CB_IDLE;
>  		rsp->gp_state = GP_IDLE;
>  	}
>  	spin_unlock_irqrestore(&rsp->rss_lock, flags);
>  }
>  
>  /**
> - * rcu_sync_exit() - Allow readers back onto fast patch after grace period
> + * rcu_sync_enter() - Force readers onto slowpath
> + * @rsp: Pointer to rcu_sync structure to use for synchronization
> + *
> + * This function is used by updaters who need readers to make use of
> + * a slowpath during the update.  After this function returns, all
> + * subsequent calls to rcu_sync_is_idle() will return false, which
> + * tells readers to stay off their fastpaths.  A later call to
> + * rcu_sync_exit() re-enables reader slowpaths.
> + *
> + * When called in isolation, rcu_sync_enter() must wait for a grace
> + * period, however, closely spaced calls to rcu_sync_enter() can
> + * optimize away the grace-period wait via a state machine implemented
> + * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
> + */
> +void rcu_sync_enter(struct rcu_sync *rsp)
> +{
> +	int gp_state;
> +
> +	spin_lock_irq(&rsp->rss_lock);
> +	gp_state = rsp->gp_state;
> +	if (gp_state == GP_IDLE) {
> +		rsp->gp_state = GP_ENTER;
> +		WARN_ON_ONCE(rsp->gp_count);
> +		/*
> +		 * Note that we could simply do rcu_sync_call(rsp) here and
> +		 * avoid the "if (gp_state == GP_IDLE)" block below.
> +		 *
> +		 * However, synchronize_rcu() can be faster if rcu_expedited
> +		 * or rcu_blocking_is_gp() is true.
> +		 *
> +		 * Another reason is that we can't wait for rcu callback if
> +		 * we are called at early boot time but this shouldn't happen.
> +		 */
> +	}
> +	rsp->gp_count++;
> +	spin_unlock_irq(&rsp->rss_lock);
> +
> +	if (gp_state == GP_IDLE) {
> +		/*
> +		 * See the comment above, this simply does the "synchronous"
> +		 * call_rcu(rcu_sync_func) which does GP_ENTER -> GP_PASSED.
> +		 */
> +		synchronize_rcu();
> +		rcu_sync_func(&rsp->cb_head);
> +		/* Not really needed, wait_event() would see GP_PASSED. */
> +		return;
> +	}
> +
> +	wait_event(rsp->gp_wait, rsp->gp_state >= GP_PASSED);
> +}
> +
> +/**
> + * rcu_sync_exit() - Allow readers back onto fast path after grace period
>   * @rsp: Pointer to rcu_sync structure to use for synchronization
>   *
>   * This function is used by updaters who have completed, and can therefore
> @@ -146,13 +167,16 @@ static void rcu_sync_func(struct rcu_head *rhp)
>   */
>  void rcu_sync_exit(struct rcu_sync *rsp)
>  {
> +	WARN_ON_ONCE(rsp->gp_state == GP_IDLE);
> +	WARN_ON_ONCE(rsp->gp_count == 0);
> +
>  	spin_lock_irq(&rsp->rss_lock);
>  	if (!--rsp->gp_count) {
> -		if (rsp->cb_state == CB_IDLE) {
> -			rsp->cb_state = CB_PENDING;
> -			call_rcu(&rsp->cb_head, rcu_sync_func);
> -		} else if (rsp->cb_state == CB_PENDING) {
> -			rsp->cb_state = CB_REPLAY;
> +		if (rsp->gp_state == GP_PASSED) {
> +			rsp->gp_state = GP_EXIT;
> +			rcu_sync_call(rsp);
> +		} else if (rsp->gp_state == GP_EXIT) {
> +			rsp->gp_state = GP_REPLAY;
>  		}
>  	}
>  	spin_unlock_irq(&rsp->rss_lock);
> @@ -164,18 +188,19 @@ void rcu_sync_exit(struct rcu_sync *rsp)
>   */
>  void rcu_sync_dtor(struct rcu_sync *rsp)
>  {
> -	int cb_state;
> +	int gp_state;
>  
>  	WARN_ON_ONCE(rsp->gp_count);
> +	WARN_ON_ONCE(rsp->gp_state == GP_PASSED);
>  
>  	spin_lock_irq(&rsp->rss_lock);
> -	if (rsp->cb_state == CB_REPLAY)
> -		rsp->cb_state = CB_PENDING;
> -	cb_state = rsp->cb_state;
> +	if (rsp->gp_state == GP_REPLAY)
> +		rsp->gp_state = GP_EXIT;
> +	gp_state = rsp->gp_state;
>  	spin_unlock_irq(&rsp->rss_lock);
>  
> -	if (cb_state != CB_IDLE) {
> +	if (gp_state != GP_IDLE) {
>  		rcu_barrier();
> -		WARN_ON_ONCE(rsp->cb_state != CB_IDLE);
> +		WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
>  	}
>  }
> -- 
> 2.5.0
> 
>