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-rw-r--r--kernel/rcu/sync.c206
1 files changed, 206 insertions, 0 deletions
diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
new file mode 100644
index 0000000000..e550f97779
--- /dev/null
+++ b/kernel/rcu/sync.c
@@ -0,0 +1,206 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * RCU-based infrastructure for lightweight reader-writer locking
+ *
+ * Copyright (c) 2015, Red Hat, Inc.
+ *
+ * Author: Oleg Nesterov <oleg@redhat.com>
+ */
+
+#include <linux/rcu_sync.h>
+#include <linux/sched.h>
+
+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
+ */
+void rcu_sync_init(struct rcu_sync *rsp)
+{
+ memset(rsp, 0, sizeof(*rsp));
+ init_waitqueue_head(&rsp->gp_wait);
+}
+
+/**
+ * rcu_sync_enter_start - Force readers onto slow path for multiple updates
+ * @rsp: Pointer to rcu_sync structure to use for synchronization
+ *
+ * Must be called after rcu_sync_init() and before first use.
+ *
+ * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}()
+ * pairs turn into NO-OPs.
+ */
+void rcu_sync_enter_start(struct rcu_sync *rsp)
+{
+ rsp->gp_count++;
+ rsp->gp_state = GP_PASSED;
+}
+
+
+static void rcu_sync_func(struct rcu_head *rhp);
+
+static void rcu_sync_call(struct rcu_sync *rsp)
+{
+ call_rcu_hurry(&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 *rhp)
+{
+ struct rcu_sync *rsp = container_of(rhp, 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(); everybody
+ * 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
+ *
+ * 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 fastpaths.
+ *
+ * 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
+ * now allow readers to make use of their fastpaths after a grace period
+ * has elapsed. After this grace period has completed, all subsequent
+ * calls to rcu_sync_is_idle() will return true, which tells readers that
+ * they can once again use their fastpaths.
+ */
+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->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);
+}
+
+/**
+ * rcu_sync_dtor() - Clean up an rcu_sync structure
+ * @rsp: Pointer to rcu_sync structure to be cleaned up
+ */
+void rcu_sync_dtor(struct rcu_sync *rsp)
+{
+ int gp_state;
+
+ 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->gp_state == GP_REPLAY)
+ WRITE_ONCE(rsp->gp_state, GP_EXIT);
+ gp_state = rsp->gp_state;
+ spin_unlock_irq(&rsp->rss_lock);
+
+ if (gp_state != GP_IDLE) {
+ rcu_barrier();
+ WARN_ON_ONCE(rsp->gp_state != GP_IDLE);
+ }
+}