rcu: Move synchronize_sched_expedited() to rcutree.c

Now that TREE_RCU and TREE_PREEMPT_RCU no longer do anything different
for the single-CPU case, there is no need for multiple definitions of
synchronize_sched_expedited().  It is no longer in any sense a plug-in,
so move it from kernel/rcutree_plugin.h to kernel/rcutree.c.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 117 insertions, 0 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index bcf7db2f2fd2..05470d4caba3 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -50,6 +50,8 @@
 #include <linux/wait.h>
 #include <linux/kthread.h>
 #include <linux/prefetch.h>
+#include <linux/delay.h>
+#include <linux/stop_machine.h>
 
 #include "rcutree.h"
 #include <trace/events/rcu.h>
@@ -1918,6 +1920,121 @@ void synchronize_rcu_bh(void)
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
 
+static atomic_t sync_sched_expedited_started = ATOMIC_INIT(0);
+static atomic_t sync_sched_expedited_done = ATOMIC_INIT(0);
+
+static int synchronize_sched_expedited_cpu_stop(void *data)
+{
+        /*
+         * There must be a full memory barrier on each affected CPU
+         * between the time that try_stop_cpus() is called and the
+         * time that it returns.
+         *
+         * In the current initial implementation of cpu_stop, the
+         * above condition is already met when the control reaches
+         * this point and the following smp_mb() is not strictly
+         * necessary.  Do smp_mb() anyway for documentation and
+         * robustness against future implementation changes.
+         */
+        smp_mb(); /* See above comment block. */
+        return 0;
+}
+
+/*
+ * Wait for an rcu-sched grace period to elapse, but use "big hammer"
+ * approach to force grace period to end quickly.  This consumes
+ * significant time on all CPUs, and is thus not recommended for
+ * any sort of common-case code.
+ *
+ * Note that it is illegal to call this function while holding any
+ * lock that is acquired by a CPU-hotplug notifier.  Failing to
+ * observe this restriction will result in deadlock.
+ *
+ * This implementation can be thought of as an application of ticket
+ * locking to RCU, with sync_sched_expedited_started and
+ * sync_sched_expedited_done taking on the roles of the halves
+ * of the ticket-lock word.  Each task atomically increments
+ * sync_sched_expedited_started upon entry, snapshotting the old value,
+ * then attempts to stop all the CPUs.  If this succeeds, then each
+ * CPU will have executed a context switch, resulting in an RCU-sched
+ * grace period.  We are then done, so we use atomic_cmpxchg() to
+ * update sync_sched_expedited_done to match our snapshot -- but
+ * only if someone else has not already advanced past our snapshot.
+ *
+ * On the other hand, if try_stop_cpus() fails, we check the value
+ * of sync_sched_expedited_done.  If it has advanced past our
+ * initial snapshot, then someone else must have forced a grace period
+ * some time after we took our snapshot.  In this case, our work is
+ * done for us, and we can simply return.  Otherwise, we try again,
+ * but keep our initial snapshot for purposes of checking for someone
+ * doing our work for us.
+ *
+ * If we fail too many times in a row, we fall back to synchronize_sched().
+ */
+void synchronize_sched_expedited(void)
+{
+        int firstsnap, s, snap, trycount = 0;
+
+        /* Note that atomic_inc_return() implies full memory barrier. */
+        firstsnap = snap = atomic_inc_return(&sync_sched_expedited_started);
+        get_online_cpus();
+        WARN_ON_ONCE(cpu_is_offline(smp_processor_id()));
+
+        /*
+         * Each pass through the following loop attempts to force a
+         * context switch on each CPU.
+         */
+        while (try_stop_cpus(cpu_online_mask,
+                             synchronize_sched_expedited_cpu_stop,
+                             NULL) == -EAGAIN) {
+                put_online_cpus();
+
+                /* No joy, try again later.  Or just synchronize_sched(). */
+                if (trycount++ < 10)
+                        udelay(trycount * num_online_cpus());
+                else {
+                        synchronize_sched();
+                        return;
+                }
+
+                /* Check to see if someone else did our work for us. */
+                s = atomic_read(&sync_sched_expedited_done);
+                if (UINT_CMP_GE((unsigned)s, (unsigned)firstsnap)) {
+                        smp_mb(); /* ensure test happens before caller kfree */
+                        return;
+                }
+
+                /*
+                 * Refetching sync_sched_expedited_started allows later
+                 * callers to piggyback on our grace period.  We subtract
+                 * 1 to get the same token that the last incrementer got.
+                 * We retry after they started, so our grace period works
+                 * for them, and they started after our first try, so their
+                 * grace period works for us.
+                 */
+                get_online_cpus();
+                snap = atomic_read(&sync_sched_expedited_started);
+                smp_mb(); /* ensure read is before try_stop_cpus(). */
+        }
+
+        /*
+         * Everyone up to our most recent fetch is covered by our grace
+         * period.  Update the counter, but only if our work is still
+         * relevant -- which it won't be if someone who started later
+         * than we did beat us to the punch.
+         */
+        do {
+                s = atomic_read(&sync_sched_expedited_done);
+                if (UINT_CMP_GE((unsigned)s, (unsigned)snap)) {
+                        smp_mb(); /* ensure test happens before caller kfree */
+                        break;
+                }
+        } while (atomic_cmpxchg(&sync_sched_expedited_done, s, snap) != s);
+
+        put_online_cpus();
+}
+EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
+
 /*
  * Check to see if there is any immediate RCU-related work to be done
  * by the current CPU, for the specified type of RCU, returning 1 if so.