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rcu/sync: Simplify the state machine
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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 <[email protected]>
[ paulmck: While here, apply READ_ONCE() and WRITE_ONCE() to ->gp_state. ]
[ paulmck: Tweaks to make htmldocs happy. (Reported by kbuild test robot.) ]
Signed-off-by: Paul E. McKenney <[email protected]>
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@oleg-nesterov
oleg-nesterov authored and Paul E. McKenney committed May 28, 2019
1 parent 3f2947b commit 89da3b9
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Showing 2 changed files with 96 additions and 73 deletions.
4 changes: 1 addition & 3 deletions include/linux/rcu_sync.h
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Original file line number Diff line number Diff line change
Expand Up @@ -19,7 +19,6 @@ struct rcu_sync {
int gp_count;
wait_queue_head_t gp_wait;

int cb_state;
struct rcu_head cb_head;
};

Expand All @@ -36,7 +35,7 @@ static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
!rcu_read_lock_bh_held() &&
!rcu_read_lock_sched_held(),
"suspicious rcu_sync_is_idle() usage");
return !rsp->gp_state; /* GP_IDLE */
return !READ_ONCE(rsp->gp_state); /* GP_IDLE */
}

extern void rcu_sync_init(struct rcu_sync *);
Expand All @@ -49,7 +48,6 @@ 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) \
Expand Down
165 changes: 95 additions & 70 deletions kernel/rcu/sync.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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)
{
Expand All @@ -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 *rhp);

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.
*
Expand All @@ -107,35 +75,88 @@ static void rcu_sync_func(struct rcu_head *rhp)
struct rcu_sync *rsp = container_of(rhp, 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(READ_ONCE(rsp->gp_state) == GP_IDLE);
WARN_ON_ONCE(READ_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) {
WRITE_ONCE(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);
WRITE_ONCE(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;
WRITE_ONCE(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) {
WRITE_ONCE(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, READ_ONCE(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
Expand All @@ -146,13 +167,16 @@ static void rcu_sync_func(struct rcu_head *rhp)
*/
void rcu_sync_exit(struct rcu_sync *rsp)
{
WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE);
WARN_ON_ONCE(READ_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) {
WRITE_ONCE(rsp->gp_state, GP_EXIT);
rcu_sync_call(rsp);
} else if (rsp->gp_state == GP_EXIT) {
WRITE_ONCE(rsp->gp_state, GP_REPLAY);
}
}
spin_unlock_irq(&rsp->rss_lock);
Expand All @@ -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(READ_ONCE(rsp->gp_count));
WARN_ON_ONCE(READ_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)
WRITE_ONCE(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);
}
}

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