rcu: Flip ->completed only once per SRCU grace period

This is an optimization of the SRCU grace period.  To guard against
preempted readers with old values of the counter, it suffices to scan the
old counters once more, then flip ->completed only one time.  The reason
this works is that the old readers must have incremented the old set of
counters (if they have not yet incremented, then their critical section
starts after this grace period, so they may be safely ignored).

This commit therefore optimizes the second flip out in favor of a simple
rescan.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/srcu.c

@@ -249,26 +249,12 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
  */
 #define SYNCHRONIZE_SRCU_READER_DELAY 5
 
-/*
- * Flip the readers' index by incrementing ->completed, then wait
- * until there are no more readers using the counters referenced by
- * the old index value.  (Recall that the index is the bottom bit
- * of ->completed.)
- *
- * Of course, it is possible that a reader might be delayed for the
- * full duration of flip_idx_and_wait() between fetching the
- * index and incrementing its counter.  This possibility is handled
- * by __synchronize_srcu() invoking flip_idx_and_wait() twice.
- */
-static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
+static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
 {
-	int idx;
 	int trycount = 0;
 
-	idx = sp->completed++ & 0x1;
-
 	/*
-	 * If a reader fetches the index before the above increment,
+	 * If a reader fetches the index before the ->completed increment,
 	 * but increments its counter after srcu_readers_active_idx_check()
 	 * sums it, then smp_mb() D will pair with __srcu_read_lock()'s
 	 * smp_mb() B to ensure that the SRCU read-side critical section
@@ -298,17 +284,38 @@ static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
 	 * sees srcu_read_unlock()'s counter decrement, then any
 	 * of the current task's subsequent code will happen after
 	 * that SRCU read-side critical section.
+	 *
+	 * It also ensures the order between the above waiting and
+	 * the next flipping.
 	 */
 	smp_mb(); /* E */
 }
 
+/*
+ * Flip the readers' index by incrementing ->completed, then wait
+ * until there are no more readers using the counters referenced by
+ * the old index value.  (Recall that the index is the bottom bit
+ * of ->completed.)
+ *
+ * Of course, it is possible that a reader might be delayed for the
+ * full duration of flip_idx_and_wait() between fetching the
+ * index and incrementing its counter.  This possibility is handled
+ * by the next __synchronize_srcu() invoking wait_idx() for such readers
+ * before starting a new grace period.
+ */
+static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
+{
+	int idx;
+
+	idx = sp->completed++ & 0x1;
+	wait_idx(sp, idx, expedited);
+}
+
 /*
  * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
  */
 static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
 {
-	int idx = 0;
-
 	rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
 			   !lock_is_held(&rcu_bh_lock_map) &&
 			   !lock_is_held(&rcu_lock_map) &&
@@ -318,27 +325,40 @@ static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
 	mutex_lock(&sp->mutex);
 
 	/*
-	 * If there were no helpers, then we need to do two flips of
-	 * the index.  The first flip is required if there are any
-	 * outstanding SRCU readers even if there are no new readers
-	 * running concurrently with the first counter flip.
+	 * Suppose that during the previous grace period, a reader
+	 * picked up the old value of the index, but did not increment
+	 * its counter until after the previous instance of
+	 * __synchronize_srcu() did the counter summation and recheck.
+	 * That previous grace period was OK because the reader did
+	 * not start until after the grace period started, so the grace
+	 * period was not obligated to wait for that reader.
 	 *
-	 * The second flip is required when a new reader picks up
-	 * the old value of the index, but does not increment its
-	 * counter until after its counters is summed/rechecked by
-	 * srcu_readers_active_idx_check().  In this case, the current SRCU
-	 * grace period would be OK because the SRCU read-side critical
-	 * section started after this SRCU grace period started, so the
-	 * grace period is not required to wait for the reader.
+	 * However, the current SRCU grace period does have to wait for
+	 * that reader.  This is handled by invoking wait_idx() on the
+	 * non-active set of counters (hence sp->completed - 1).  Once
+	 * wait_idx() returns, we know that all readers that picked up
+	 * the old value of ->completed and that already incremented their
+	 * counter will have completed.
 	 *
-	 * However, the next SRCU grace period would be waiting for the
-	 * other set of counters to go to zero, and therefore would not
-	 * wait for the reader, which would be very bad.  To avoid this
-	 * bad scenario, we flip and wait twice, clearing out both sets
-	 * of counters.
+	 * But what about readers that picked up the old value of
+	 * ->completed, but -still- have not managed to increment their
+	 * counter?  We do not need to wait for those readers, because
+	 * they will have started their SRCU read-side critical section
+	 * after the current grace period starts.
+	 *
+	 * Because it is unlikely that readers will be preempted between
+	 * fetching ->completed and incrementing their counter, wait_idx()
+	 * will normally not need to wait.
+	 */
+	wait_idx(sp, (sp->completed - 1) & 0x1, expedited);
+
+	/*
+	 * Now that wait_idx() has waited for the really old readers,
+	 * invoke flip_idx_and_wait() to flip the counter and wait
+	 * for current SRCU readers.
 	 */
-	for (; idx < 2; idx++)
 	flip_idx_and_wait(sp, expedited);
+
 	mutex_unlock(&sp->mutex);
 }