[PATCH RFC tip/core/rcu] rcu: direct algorithmic SRCU implementation

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

The current implementation of synchronize_srcu_expedited() can cause
severe OS jitter due to its use of synchronize_sched(), which in turn
invokes try_stop_cpus(), which causes each CPU to be sent an IPI.
This can result in severe performance degradation for real-time workloads
and especially for short-interation-length HPC workloads.  Furthermore,
because only one instance of try_stop_cpus() can be making forward progress
at a given time, only one instance of synchronize_srcu_expedited() can
make forward progress at a time, even if they are all operating on
distinct srcu_struct structures.

This commit, inspired by an earlier implementation by Peter Zijlstra
(https://lkml.org/lkml/2012/1/31/211) and by further offline discussions,
takes a strictly algorithmic bits-in-memory approach.  This has the
disadvantage of requiring one explicit memory-barrier instruction in
each of srcu_read_lock() and srcu_read_unlock(), but on the other hand
completely dispenses with OS jitter and furthermore allows SRCU to be
used freely by CPUs that RCU believes to be idle or offline.

The update-side implementation handles the single read-side memory
barrier by rechecking the per-CPU counters after summing them and
by running through the update-side state machine twice.

This implementation has passed moderate rcutorture testing on both 32-bit
x86 and 64-bit Power.  A call_srcu() function will be present in a later
version of this patch.

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

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index d3d5fa5..a478c8e 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -31,13 +31,19 @@
 #include <linux/rcupdate.h>
 
 struct srcu_struct_array {
-	int c[2];
+	unsigned long c[2];
 };
 
+/* Bit definitions for field ->c above and ->snap below. */
+#define SRCU_USAGE_BITS		2
+#define SRCU_REF_MASK		(ULONG_MAX >> SRCU_USAGE_BITS)
+#define SRCU_USAGE_COUNT	(SRCU_REF_MASK + 1)
+
 struct srcu_struct {
-	int completed;
+	unsigned completed;
 	struct srcu_struct_array __percpu *per_cpu_ref;
 	struct mutex mutex;
+	unsigned long snap[NR_CPUS];
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 	struct lockdep_map dep_map;
 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index e0fe148..3d99162 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -620,7 +620,7 @@ static int srcu_torture_stats(char *page)
 	cnt += sprintf(&page[cnt], "%s%s per-CPU(idx=%d):",
 		       torture_type, TORTURE_FLAG, idx);
 	for_each_possible_cpu(cpu) {
-		cnt += sprintf(&page[cnt], " %d(%d,%d)", cpu,
+		cnt += sprintf(&page[cnt], " %d(%lu,%lu)", cpu,
 			       per_cpu_ptr(srcu_ctl.per_cpu_ref, cpu)->c[!idx],
 			       per_cpu_ptr(srcu_ctl.per_cpu_ref, cpu)->c[idx]);
 	}
diff --git a/kernel/srcu.c b/kernel/srcu.c
index ba35f3a..540671e 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -73,19 +73,102 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /*
- * srcu_readers_active_idx -- returns approximate number of readers
- *	active on the specified rank of per-CPU counters.
+ * Returns approximate number of readers active on the specified rank
+ * of per-CPU counters.  Also snapshots each counter's value in the
+ * corresponding element of sp->snap[] for later use validating
+ * the sum.
  */
+static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
+{
+	int cpu;
+	unsigned long sum = 0;
+	unsigned long t;
 
-static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
+	for_each_possible_cpu(cpu) {
+		t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
+		sum += t;
+		sp->snap[cpu] = t;
+	}
+	return sum & SRCU_REF_MASK;
+}
+
+/*
+ * To be called from the update side after an index flip.  Returns true
+ * if the modulo sum of the counters is stably zero, false if there is
+ * some possibility of non-zero.
+ */
+static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
 {
 	int cpu;
-	int sum;
 
-	sum = 0;
+	/*
+	 * Note that srcu_readers_active_idx() can incorrectly return
+	 * zero even though there is a pre-existing reader throughout.
+	 * To see this, suppose that task A is in a very long SRCU
+	 * read-side critical section that started on CPU 0, and that
+	 * no other reader exists, so that the modulo sum of the counters
+	 * is equal to one.  Then suppose that task B starts executing
+	 * srcu_readers_active_idx(), summing up to CPU 1, and then that
+	 * task C starts reading on CPU 0, so that its increment is not
+	 * summed, but finishes reading on CPU 2, so that its decrement
+	 * -is- summed.  Then when task B completes its sum, it will
+	 * incorrectly get zero, despite the fact that task A has been
+	 * in its SRCU read-side critical section the whole time.
+	 *
+	 * We therefore do a validation step should srcu_readers_active_idx()
+	 * return zero.
+	 */
+	if (srcu_readers_active_idx(sp, idx) != 0)
+		return false;
+
+	/*
+	 * Since the caller recently flipped ->completed, we can see at
+	 * most one increment of each CPU's counter from this point
+	 * forward.  The reason for this is that the reader CPU must have
+	 * fetched the index before srcu_readers_active_idx checked
+	 * that CPU's counter, but not yet incremented its counter.
+	 * Its eventual counter increment will follow the read in
+	 * srcu_readers_active_idx(), and that increment is immediately
+	 * followed by smp_mb() B.  Because smp_mb() D is between
+	 * the ->completed flip and srcu_readers_active_idx()'s read,
+	 * that CPU's subsequent load of ->completed must see the new
+	 * value, and therefore increment the counter in the other rank.
+	 */
+	smp_mb(); /* A */
+
+	/*
+	 * Now, we check the ->snap array that srcu_readers_active_idx()
+	 * filled in from the per-CPU counter values.  Since both
+	 * __srcu_read_lock() and __srcu_read_unlock() increment the
+	 * upper bits of the per-CPU counter, an increment/decrement
+	 * pair will change the value of the counter.  Since there is
+	 * only one possible increment, the only way to wrap the counter
+	 * is to have a huge number of counter decrements, which requires
+	 * a huge number of tasks and huge SRCU read-side critical-section
+	 * nesting levels, even on 32-bit systems.
+	 *
+	 * All of the ways of confusing the readings require that the scan
+	 * in srcu_readers_active_idx() see the read-side task's decrement,
+	 * but not its increment.  However, between that decrement and
+	 * increment are smb_mb() B and C.  Either or both of these pair
+	 * with smp_mb() A above to ensure that the scan below will see
+	 * the read-side tasks's increment, thus noting a difference in
+	 * the counter values between the two passes.
+	 *
+	 * Therefore, if srcu_readers_active_idx() returned zero, and
+	 * none of the counters changed, we know that the zero was the
+	 * correct sum.
+	 *
+	 * Of course, it is possible that a task might be delayed
+	 * for a very long time in __srcu_read_lock() after fetching
+	 * the index but before incrementing its counter.  This
+	 * possibility will be dealt with in __synchronize_srcu().
+	 */
 	for_each_possible_cpu(cpu)
-		sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
-	return sum;
+		if (sp->snap[cpu] !=
+		    ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]))
+			return false;  /* False zero reading! */
+	return true;
 }
 
 /**
@@ -131,10 +214,11 @@ int __srcu_read_lock(struct srcu_struct *sp)
 	int idx;
 
 	preempt_disable();
-	idx = sp->completed & 0x1;
-	barrier();  /* ensure compiler looks -once- at sp->completed. */
-	per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
-	srcu_barrier();  /* ensure compiler won't misorder critical section. */
+	idx = rcu_dereference_index_check(sp->completed,
+					  rcu_read_lock_sched_held()) & 0x1;
+	ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]) +=
+		SRCU_USAGE_COUNT + 1;
+	smp_mb(); /* B */  /* Avoid leaking the critical section. */
 	preempt_enable();
 	return idx;
 }
@@ -149,8 +233,9 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
 void __srcu_read_unlock(struct srcu_struct *sp, int idx)
 {
 	preempt_disable();
-	srcu_barrier();  /* ensure compiler won't misorder critical section. */
-	per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
+	smp_mb(); /* C */  /* Avoid leaking the critical section. */
+	ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]) +=
+		SRCU_USAGE_COUNT - 1;
 	preempt_enable();
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
@@ -163,12 +248,65 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
  * we repeatedly block for 1-millisecond time periods.  This approach
  * has done well in testing, so there is no need for a config parameter.
  */
-#define SYNCHRONIZE_SRCU_READER_DELAY 10
+#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)
+{
+	int idx;
+	int trycount = 0;
+
+	idx = sp->completed++ & 0x1;
+
+	/*
+	 * If a reader fetches the index before the above 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
+	 * will see any updates that the current task performed before its
+	 * call to synchronize_srcu(), or to synchronize_srcu_expedited(),
+	 * as the case may be.
+	 */
+	smp_mb(); /* D */
+
+	/*
+	 * SRCU read-side critical sections are normally short, so wait
+	 * a small amount of time before possibly blocking.
+	 */
+	if (!srcu_readers_active_idx_check(sp, idx)) {
+		udelay(SYNCHRONIZE_SRCU_READER_DELAY);
+		while (!srcu_readers_active_idx_check(sp, idx)) {
+			if (expedited && ++ trycount < 10)
+				udelay(SYNCHRONIZE_SRCU_READER_DELAY);
+			else
+				schedule_timeout_interruptible(1);
+		}
+	}
+
+	/*
+	 * The following smp_mb() E pairs with srcu_read_unlock()'s
+	 * smp_mb C to ensure that if srcu_readers_active_idx_check()
+	 * 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.
+	 */
+	smp_mb(); /* E */
+}
 
 /*
  * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
  */
-static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
+static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
 {
 	int idx;
 
@@ -178,90 +316,51 @@ static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
 			   !lock_is_held(&rcu_sched_lock_map),
 			   "Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
 
-	idx = sp->completed;
+	idx = ACCESS_ONCE(sp->completed);
 	mutex_lock(&sp->mutex);
 
 	/*
 	 * Check to see if someone else did the work for us while we were
-	 * waiting to acquire the lock.  We need -two- advances of
+	 * waiting to acquire the lock.  We need -three- advances of
 	 * the counter, not just one.  If there was but one, we might have
 	 * shown up -after- our helper's first synchronize_sched(), thus
 	 * having failed to prevent CPU-reordering races with concurrent
-	 * srcu_read_unlock()s on other CPUs (see comment below).  So we
-	 * either (1) wait for two or (2) supply the second ourselves.
+	 * srcu_read_unlock()s on other CPUs (see comment below).  If there
+	 * was only two, we are guaranteed to have waited through only one
+	 * full index-flip phase.  So we either (1) wait for three or
+	 * (2) supply the additional ones we need.
 	 */
 
-	if ((sp->completed - idx) >= 2) {
+	if (sp->completed == idx + 2)
+		idx = 1;
+	else if (sp->completed == idx + 3) {
 		mutex_unlock(&sp->mutex);
 		return;
-	}
-
-	sync_func();  /* Force memory barrier on all CPUs. */
+	} else
+		idx = 0;
 
 	/*
-	 * The preceding synchronize_sched() ensures that any CPU that
-	 * sees the new value of sp->completed will also see any preceding
-	 * changes to data structures made by this CPU.  This prevents
-	 * some other CPU from reordering the accesses in its SRCU
-	 * read-side critical section to precede the corresponding
-	 * srcu_read_lock() -- ensuring that such references will in
-	 * fact be protected.
+	 * 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.
 	 *
-	 * So it is now safe to do the flip.
-	 */
-
-	idx = sp->completed & 0x1;
-	sp->completed++;
-
-	sync_func();  /* Force memory barrier on all CPUs. */
-
-	/*
-	 * At this point, because of the preceding synchronize_sched(),
-	 * all srcu_read_lock() calls using the old counters have completed.
-	 * Their corresponding critical sections might well be still
-	 * executing, but the srcu_read_lock() primitives themselves
-	 * will have finished executing.  We initially give readers
-	 * an arbitrarily chosen 10 microseconds to get out of their
-	 * SRCU read-side critical sections, then loop waiting 1/HZ
-	 * seconds per iteration.  The 10-microsecond value has done
-	 * very well in testing.
-	 */
-
-	if (srcu_readers_active_idx(sp, idx))
-		udelay(SYNCHRONIZE_SRCU_READER_DELAY);
-	while (srcu_readers_active_idx(sp, idx))
-		schedule_timeout_interruptible(1);
-
-	sync_func();  /* Force memory barrier on all CPUs. */
-
-	/*
-	 * The preceding synchronize_sched() forces all srcu_read_unlock()
-	 * primitives that were executing concurrently with the preceding
-	 * for_each_possible_cpu() loop to have completed by this point.
-	 * More importantly, it also forces the corresponding SRCU read-side
-	 * critical sections to have also completed, and the corresponding
-	 * references to SRCU-protected data items to be dropped.
+	 * 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.
 	 *
-	 * Note:
-	 *
-	 *	Despite what you might think at first glance, the
-	 *	preceding synchronize_sched() -must- be within the
-	 *	critical section ended by the following mutex_unlock().
-	 *	Otherwise, a task taking the early exit can race
-	 *	with a srcu_read_unlock(), which might have executed
-	 *	just before the preceding srcu_readers_active() check,
-	 *	and whose CPU might have reordered the srcu_read_unlock()
-	 *	with the preceding critical section.  In this case, there
-	 *	is nothing preventing the synchronize_sched() task that is
-	 *	taking the early exit from freeing a data structure that
-	 *	is still being referenced (out of order) by the task
-	 *	doing the srcu_read_unlock().
-	 *
-	 *	Alternatively, the comparison with "2" on the early exit
-	 *	could be changed to "3", but this increases synchronize_srcu()
-	 *	latency for bulk loads.  So the current code is preferred.
+	 * 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.
 	 */
-
+	for (; idx < 2; idx++)
+		flip_idx_and_wait(sp, expedited);
 	mutex_unlock(&sp->mutex);
 }
 
@@ -281,7 +380,7 @@ static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
  */
 void synchronize_srcu(struct srcu_struct *sp)
 {
-	__synchronize_srcu(sp, synchronize_sched);
+	__synchronize_srcu(sp, 0);
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);
 
@@ -289,18 +388,11 @@ EXPORT_SYMBOL_GPL(synchronize_srcu);
  * synchronize_srcu_expedited - Brute-force SRCU grace period
  * @sp: srcu_struct with which to synchronize.
  *
- * Wait for an SRCU grace period to elapse, but use a "big hammer"
- * approach to force the grace period to end quickly.  This consumes
- * significant time on all CPUs and is unfriendly to real-time workloads,
- * so is thus not recommended for any sort of common-case code.  In fact,
- * if you are using synchronize_srcu_expedited() in a loop, please
- * restructure your code to batch your updates, and then use a single
- * synchronize_srcu() instead.
+ * Wait for an SRCU grace period to elapse, but be more aggressive about
+ * spinning rather than blocking when waiting.
  *
  * Note that it is illegal to call this function while holding any lock
- * that is acquired by a CPU-hotplug notifier.  And yes, it is also illegal
- * to call this function from a CPU-hotplug notifier.  Failing to observe
- * these restriction will result in deadlock.  It is also illegal to call
+ * that is acquired by a CPU-hotplug notifier.  It is also illegal to call
  * synchronize_srcu_expedited() from the corresponding SRCU read-side
  * critical section; doing so will result in deadlock.  However, it is
  * perfectly legal to call synchronize_srcu_expedited() on one srcu_struct
@@ -309,7 +401,7 @@ EXPORT_SYMBOL_GPL(synchronize_srcu);
  */
 void synchronize_srcu_expedited(struct srcu_struct *sp)
 {
-	__synchronize_srcu(sp, synchronize_sched_expedited);
+	__synchronize_srcu(sp, 1);
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);