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

From: Paul E. McKenney
Date: Sun Apr 28 2019 - 18:27:08 EST

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Next in thread: Oleg Nesterov: "Re: [PATCH 1/1] rcu/sync: simplify the state machine"
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On Sat, Apr 27, 2019 at 02:02:30PM -0700, Paul E. McKenney wrote:
> 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!

And it still looks good after review, so I have pushed it. I did add
READ_ONCE() and WRITE_ONCE() to unprotected uses of ->gp_state, but
please let me know if I messed anything up.

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
> >
> >

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