Messages in this thread | | | Date | Sun, 28 Apr 2019 15:26:52 -0700 | From | "Paul E. McKenney" <> | Subject | 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@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!
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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