rcu/sync: Simplify the state machine
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>
[ paulmck: While here, apply READ_ONCE() and WRITE_ONCE() to ->gp_state. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h
index 87971e8..9b83865 100644
--- a/include/linux/rcu_sync.h
+++ b/include/linux/rcu_sync.h
@@ -19,7 +19,6 @@
int gp_count;
wait_queue_head_t gp_wait;
- int cb_state;
struct rcu_head cb_head;
};
@@ -36,7 +35,7 @@
!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 *);
@@ -49,7 +48,6 @@
.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..f72b779 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)
{
@@ -41,6 +39,70 @@
rsp->gp_state = GP_PASSED;
}
+
+static void rcu_sync_func(struct rcu_head *rcu);
+
+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 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 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.
+ *
+ * If another rcu_sync_enter() is invoked before the grace period
+ * ended, reset state to allow the next rcu_sync_exit() to let the
+ * readers back onto their fastpaths (after a grace period). If both
+ * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
+ * before the grace period ended, re-invoke call_rcu() on behalf of that
+ * 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 *rcu)
+{
+ struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head);
+ unsigned long flags;
+
+ 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) {
+ /*
+ * We're at least a GP after the GP_IDLE->GP_ENTER transition.
+ */
+ 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.
+ */
+ WRITE_ONCE(rsp->gp_state, GP_EXIT);
+ rcu_sync_call(rsp);
+ } else {
+ /*
+ * 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!.
+ */
+ WRITE_ONCE(rsp->gp_state, GP_IDLE);
+ }
+ spin_unlock_irqrestore(&rsp->rss_lock, flags);
+}
+
/**
* rcu_sync_enter() - Force readers onto slowpath
* @rsp: Pointer to rcu_sync structure to use for synchronization
@@ -58,84 +120,43 @@
*/
void rcu_sync_enter(struct rcu_sync *rsp)
{
- bool need_wait, need_sync;
+ int gp_state;
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;
+ 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);
- WARN_ON_ONCE(need_wait && need_sync);
- if (need_sync) {
+ 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();
- 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);
+ 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_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
- * 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
- * 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.
- *
- * If another rcu_sync_enter() is invoked before the grace period
- * ended, reset state to allow the next rcu_sync_exit() to let the
- * readers back onto their fastpaths (after a grace period). If both
- * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
- * before the grace period ended, re-invoke call_rcu() on behalf of that
- * 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)
-{
- 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);
-
- spin_lock_irqsave(&rsp->rss_lock, flags);
- if (rsp->gp_count) {
- /*
- * A new rcu_sync_begin() has happened; drop the callback.
- */
- rsp->cb_state = CB_IDLE;
- } else if (rsp->cb_state == CB_REPLAY) {
- /*
- * 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);
- } else {
- /*
- * We're at least a GP after 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_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 @@
*/
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);
@@ -164,18 +188,19 @@
*/
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);
}
}
