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 <o...@redhat.com>
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
>
>
