SRCU: More efficient reader counts.

SRCU uses two per-cpu counters: a nesting counter to count the number of active critical sections, and a sequence counter to ensure that the nesting counters don't change while they are being added together in srcu_readers_active_idx_check().

This patch instead uses per-cpu lock and unlock counters. Because the both counters only increase and srcu_readers_active_idx_check() reads the unlock counter before the lock counter, this achieves the same end without having to increment two different counters in srcu_read_lock(). This also saves a smp_mb() in srcu_readers_active_idx_check().

Possible bug: There is no guarantee that the lock counter won't overflow during srcu_readers_active_idx_check(), as there are no memory barriers around srcu_flip() (see comment in srcu_readers_active_idx_check() for details). However, this problem was already present before this patch.

Suggested-by Mathieu Desnoyers <[email protected]>

Signed-off-by Lance Roy <[email protected]>

---
 include/linux/srcu.h    |   4 +-
 kernel/rcu/rcutorture.c |  20 ++++++++-
 kernel/rcu/srcu.c       | 116 ++++++++++++++++++------------------------------
 3 files changed, 63 insertions(+), 77 deletions(-)

include/linux/srcu.h +2 -2

index dc8eb63..0caea34 100644
@@ -34,8 +34,8 @@
 #include <linux/workqueue.h>
 
 struct srcu_struct_array {
-	unsigned long c[2];
-	unsigned long seq[2];
+	unsigned long lock_count[2];
+	unsigned long unlock_count[2];
 };
 
 struct rcu_batch {

kernel/rcu/rcutorture.c +18 -2

index bf08fee..2450c61 100644
@@ -555,10 +555,26 @@ static void srcu_torture_stats(void)
 	pr_alert("%s%s per-CPU(idx=%d):",
 		 torture_type, TORTURE_FLAG, idx);
 	for_each_possible_cpu(cpu) {
+		unsigned long l0, l1;
+		unsigned long u0, u1;
 		long c0, c1;
+		struct srcu_struct_array *counts =
+			per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu);
 
-		c0 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[!idx];
-		c1 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[idx];
+		u0 = counts->unlock_count[!idx];
+		u1 = counts->unlock_count[idx];
+
+		/*
+		 * Make sure that a lock is always counted if the corresponding
+		 * unlock is counted.
+		 */
+		smp_rmb();
+
+		l0 = counts->lock_count[!idx];
+		l1 = counts->lock_count[idx];
+
+		c0 = (long)(l0 - u0);
+		c1 = (long)(l1 - u1);
 		pr_cont(" %d(%ld,%ld)", cpu, c0, c1);
 	}
 	pr_cont("\n");

kernel/rcu/srcu.c +43 -73

index 9b9cdd5..38e9aae 100644
@@ -141,34 +141,38 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /*
- * Returns approximate total of the readers' ->seq[] values for the
+ * Returns approximate total of the readers' ->lock_count[] values for the
  * rank of per-CPU counters specified by idx.
  */
-static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
+static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
 {
 	int cpu;
 	unsigned long sum = 0;
 	unsigned long t;
 
 	for_each_possible_cpu(cpu) {
-		t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
+		struct srcu_struct_array *cpu_counts =
+			per_cpu_ptr(sp->per_cpu_ref, cpu);
+		t = READ_ONCE(cpu_counts->lock_count[idx]);
 		sum += t;
 	}
 	return sum;
 }
 
 /*
- * Returns approximate number of readers active on the specified rank
- * of the per-CPU ->c[] counters.
+ * Returns approximate total of the readers' ->unlock_count[] values for the
+ * rank of per-CPU counters specified by idx.
  */
-static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
+static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
 {
 	int cpu;
 	unsigned long sum = 0;
 	unsigned long t;
 
 	for_each_possible_cpu(cpu) {
-		t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
+		struct srcu_struct_array *cpu_counts =
+			per_cpu_ptr(sp->per_cpu_ref, cpu);
+		t = READ_ONCE(cpu_counts->unlock_count[idx]);
 		sum += t;
 	}
 	return sum;

@@ -176,79 +180,42 @@ static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
 
 /*
  * Return true if the number of pre-existing readers is determined to
- * be stably zero.  An example unstable zero can occur if the call
- * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
- * but due to task migration, sees the corresponding __srcu_read_unlock()
- * decrement.  This can happen because srcu_readers_active_idx() takes
- * time to sum the array, and might in fact be interrupted or preempted
- * partway through the summation.
+ * be zero.
  */
 static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
 {
-	unsigned long seq;
+	unsigned long unlocks;
 
-	seq = srcu_readers_seq_idx(sp, idx);
+	unlocks = srcu_readers_unlock_idx(sp, idx);
 
 	/*
-	 * The following smp_mb() A pairs with the smp_mb() B located in
-	 * __srcu_read_lock().  This pairing ensures that if an
-	 * __srcu_read_lock() increments its counter after the summation
-	 * in srcu_readers_active_idx(), then the corresponding SRCU read-side
-	 * critical section will see any changes made prior to the start
-	 * of the current SRCU grace period.
+	 * Make sure that a lock is always counted if the corresponding unlock
+	 * is counted. Needs to be a smp_mb() as the read side may contain a
+	 * read from a variable that is written to before the synchronize_srcu()
+	 * in the write side. In this case smp_mb()s A and B act like the store
+	 * buffering pattern.
 	 *
-	 * Also, if the above call to srcu_readers_seq_idx() saw the
-	 * increment of ->seq[], then the call to srcu_readers_active_idx()
-	 * must see the increment of ->c[].
+	 * This smp_mb() also pairs with smp_mb() C to prevent writes after the
+	 * synchronize_srcu() from being executed before the grace period ends.
 	 */
 	smp_mb(); /* A */
 
 	/*
-	 * 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 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;
-
-	/*
-	 * The remainder of this function is the validation step.
-	 * The following smp_mb() D pairs with the smp_mb() C in
-	 * __srcu_read_unlock().  If the __srcu_read_unlock() was seen
-	 * by srcu_readers_active_idx() above, then any destructive
-	 * operation performed after the grace period will happen after
-	 * the corresponding SRCU read-side critical section.
+	 * If the locks are the same as the unlocks, then there must of have
+	 * been no readers on this index at some time in between. This does not
+	 * mean that there are no more readers, as one could have read the
+	 * current index but have incremented the lock counter yet.
 	 *
-	 * Note that there can be at most NR_CPUS worth of readers using
-	 * the old index, which is not enough to overflow even a 32-bit
-	 * integer.  (Yes, this does mean that systems having more than
-	 * a billion or so CPUs need to be 64-bit systems.)  Therefore,
-	 * the sum of the ->seq[] counters cannot possibly overflow.
-	 * Therefore, the only way that the return values of the two
-	 * calls to srcu_readers_seq_idx() can be equal is if there were
-	 * no increments of the corresponding rank of ->seq[] counts
-	 * in the interim.  But the missed-increment scenario laid out
-	 * above includes an increment of the ->seq[] counter by
-	 * the corresponding __srcu_read_lock().  Therefore, if this
-	 * scenario occurs, the return values from the two calls to
-	 * srcu_readers_seq_idx() will differ, and thus the validation
-	 * step below suffices.
+	 * Possible bug: There is no guarantee that there haven't been ULONG_MAX
+	 * increments of ->lock_count[] since the unlocks were counted, meaning
+	 * that this could return true even if there are still active readers.
+	 * Since there are no memory barriers around srcu_flip(), the CPU is not
+	 * required to increment ->completed before running
+	 * srcu_readers_unlock_idx(), which means that there could be an
+	 * arbitrarily large number of critical sections that execute after
+	 * srcu_readers_unlock_idx() but use the old value of ->completed.
 	 */
-	smp_mb(); /* D */
-
-	return srcu_readers_seq_idx(sp, idx) == seq;
+	return srcu_readers_lock_idx(sp, idx) == unlocks;
 }
 
 /**

@@ -266,8 +233,12 @@ static bool srcu_readers_active(struct srcu_struct *sp)
 	unsigned long sum = 0;
 
 	for_each_possible_cpu(cpu) {
-		sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
-		sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
+		struct srcu_struct_array *cpu_counts =
+			per_cpu_ptr(sp->per_cpu_ref, cpu);
+		sum += READ_ONCE(cpu_counts->lock_count[0]);
+		sum += READ_ONCE(cpu_counts->lock_count[1]);
+		sum -= READ_ONCE(cpu_counts->unlock_count[0]);
+		sum -= READ_ONCE(cpu_counts->unlock_count[1]);
 	}
 	return sum;
 }

@@ -298,9 +269,8 @@ int __srcu_read_lock(struct srcu_struct *sp)
 	int idx;
 
 	idx = READ_ONCE(sp->completed) & 0x1;
-	__this_cpu_inc(sp->per_cpu_ref->c[idx]);
+	__this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
 	smp_mb(); /* B */  /* Avoid leaking the critical section. */
-	__this_cpu_inc(sp->per_cpu_ref->seq[idx]);
 	return idx;
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock);

@@ -314,7 +284,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
 void __srcu_read_unlock(struct srcu_struct *sp, int idx)
 {
 	smp_mb(); /* C */  /* Avoid leaking the critical section. */
-	this_cpu_dec(sp->per_cpu_ref->c[idx]);
+	this_cpu_inc(sp->per_cpu_ref->unlock_count[idx]);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 

@@ -349,7 +319,7 @@ static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
 
 /*
  * Increment the ->completed counter so that future SRCU readers will
- * use the other rank of the ->c[] and ->seq[] arrays.  This allows
+ * use the other rank of the ->(un)lock_count[] arrays.  This allows
  * us to wait for pre-existing readers in a starvation-free manner.
  */
 static void srcu_flip(struct srcu_struct *sp)

Signature · 1 line

2.9.0

Quoted 11 lines · Lance Roy

SRCU uses two per-cpu counters: a nesting counter to count the number of active critical sections, and a sequence counter to ensure that the nesting counters don't change while they are being added together in srcu_readers_active_idx_check().

This patch instead uses per-cpu lock and unlock counters. Because the both counters only increase and srcu_readers_active_idx_check() reads the unlock counter before the lock counter, this achieves the same end without having to increment two different counters in srcu_read_lock(). This also saves a smp_mb() in srcu_readers_active_idx_check().

Quoted 4 lines

Possible bug: There is no guarantee that the lock counter won't overflow during srcu_readers_active_idx_check(), as there are no memory barriers around srcu_flip() (see comment in srcu_readers_active_idx_check() for details). However, this problem was already present before this patch.

This patch differs from the previous one in a few (good) code-style changes, comment changes, and of course the commit log. Good. Once discussion converges, I will apply the agreed-upon commit, which might well be this one.

However, let's first take a look at the overflow issue.

If a given program could have ULONG_MAX or more readers at any given time, there would of course be overflow. However, each read must have an srcu_read_lock() outstanding, and the resulting four-byte return value must be stored somewhere. Because the full address space is at most ULONG_MAX, the maximum number of outstanding readers is at most ULONG_MAX/4, even in the degenerate case where a single CPU/task invokes srcu_read_lock() in a tight loop. And even this assumes that the entire address space can somehow be devoted to srcu_read_lock() return values. ULONG_MAX/4 is therefore a hard upper bound on the number of outstanding SRCU readers.

Now srcu_readers_active_idx_check() checks for strict equality between the number of locks and unlocks, so we can in theory tolerate ULONG_MAX-1 readers. So, the question is whether ULONG_MAX/4 readers can result in the updater seeing ULONG_MAX reads, due to memory misordering and other issues.

Because there are no memory barriers surrounding srcu_flip(), the updater could miss an extremely large number of srcu_read_unlock()s. However, each missed srcu_read_unlock() must have a corresponding srcu_read_lock(), and there is a full memory barrier between between the srcu_flip() and the read of the lock count. There is also a full barrier between any srcu_read_lock()'s increment of the lock count and that CPU's/task's next srcu_read_lock()'s fetch of the index. Therefore, if the updater misses counting a given srcu_read_lock(), that CPU's/task's next srcu_read_lock() must see the new value of the index. Because srcu_read_lock() disables preemption across the index fetch and the lock increment, there can be at most NR_CPUS-1 srcu_read_lock() calls that missed the recent srcu_flip()'s update to the index. (I said NR_CPUS earlier, but Mathieu is correct in pointing out that srcu_flip() has to run somewhere.)

The maximum number of locks that the updater can see is therefore:

o ULONG_MAX/4 for a full set of missed srcu_read_unlock()s.

o ULONG_MAX/4 for a full set of srcu_read_lock()s.

o NR_CPUS-1 for a full set of subsequent srcu_read_lock()s that
missed the flip.

This totals to ULONG_MAX/2+NR_CPUS-1. So as long as there are no more than ULONG_MAX/2 CPUs, we should be good. And given that the biggest system I have hard evidence of is 4K CPUs, we have ample headrooom compared to the ~2G value of ULONG_MAX/2, even on 32-bit systems.

So, what am I missing here?

Thanx, Paul

Quoted 257 lines

Suggested-by Mathieu Desnoyers <[email protected]>

Signed-off-by Lance Roy <[email protected]>

---
 include/linux/srcu.h    |   4 +-
 kernel/rcu/rcutorture.c |  20 ++++++++-
 kernel/rcu/srcu.c       | 116 ++++++++++++++++++------------------------------
 3 files changed, 63 insertions(+), 77 deletions(-)

include/linux/srcu.h +2 -2

index dc8eb63..0caea34 100644
@@ -34,8 +34,8 @@
 #include <linux/workqueue.h>
 
 struct srcu_struct_array {
-	unsigned long c[2];
-	unsigned long seq[2];
+	unsigned long lock_count[2];
+	unsigned long unlock_count[2];
 };
 
 struct rcu_batch {

kernel/rcu/rcutorture.c +18 -2

index bf08fee..2450c61 100644
@@ -555,10 +555,26 @@ static void srcu_torture_stats(void)
 	pr_alert("%s%s per-CPU(idx=%d):",
 		 torture_type, TORTURE_FLAG, idx);
 	for_each_possible_cpu(cpu) {
+		unsigned long l0, l1;
+		unsigned long u0, u1;
 		long c0, c1;
+		struct srcu_struct_array *counts =
+			per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu);
 
-		c0 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[!idx];
-		c1 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[idx];
+		u0 = counts->unlock_count[!idx];
+		u1 = counts->unlock_count[idx];
+
+		/*
+		 * Make sure that a lock is always counted if the corresponding
+		 * unlock is counted.
+		 */
+		smp_rmb();
+
+		l0 = counts->lock_count[!idx];
+		l1 = counts->lock_count[idx];
+
+		c0 = (long)(l0 - u0);
+		c1 = (long)(l1 - u1);
 		pr_cont(" %d(%ld,%ld)", cpu, c0, c1);
 	}
 	pr_cont("\n");

kernel/rcu/srcu.c +34 -67

index 9b9cdd5..38e9aae 100644
@@ -141,34 +141,38 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /*
- * Returns approximate total of the readers' ->seq[] values for the
+ * Returns approximate total of the readers' ->lock_count[] values for the
  * rank of per-CPU counters specified by idx.
  */
-static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
+static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
 {
 	int cpu;
 	unsigned long sum = 0;
 	unsigned long t;
 
 	for_each_possible_cpu(cpu) {
-		t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
+		struct srcu_struct_array *cpu_counts =
+			per_cpu_ptr(sp->per_cpu_ref, cpu);
+		t = READ_ONCE(cpu_counts->lock_count[idx]);
 		sum += t;
 	}
 	return sum;
 }
 
 /*
- * Returns approximate number of readers active on the specified rank
- * of the per-CPU ->c[] counters.
+ * Returns approximate total of the readers' ->unlock_count[] values for the
+ * rank of per-CPU counters specified by idx.
  */
-static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
+static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
 {
 	int cpu;
 	unsigned long sum = 0;
 	unsigned long t;
 
 	for_each_possible_cpu(cpu) {
-		t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
+		struct srcu_struct_array *cpu_counts =
+			per_cpu_ptr(sp->per_cpu_ref, cpu);
+		t = READ_ONCE(cpu_counts->unlock_count[idx]);
 		sum += t;
 	}
 	return sum;

@@ -176,79 +180,42 @@ static unsigned long srcu_readers_active_idx(struct
 srcu_struct *sp, int idx)
 
 /*
  * Return true if the number of pre-existing readers is determined to
- * be stably zero.  An example unstable zero can occur if the call
- * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
- * but due to task migration, sees the corresponding __srcu_read_unlock()
- * decrement.  This can happen because srcu_readers_active_idx() takes
- * time to sum the array, and might in fact be interrupted or preempted
- * partway through the summation.
+ * be zero.
  */
 static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
 {
-	unsigned long seq;
+	unsigned long unlocks;
 
-	seq = srcu_readers_seq_idx(sp, idx);
+	unlocks = srcu_readers_unlock_idx(sp, idx);
 
 	/*
-	 * The following smp_mb() A pairs with the smp_mb() B located in
-	 * __srcu_read_lock().  This pairing ensures that if an
-	 * __srcu_read_lock() increments its counter after the summation
-	 * in srcu_readers_active_idx(), then the corresponding SRCU read-side
-	 * critical section will see any changes made prior to the start
-	 * of the current SRCU grace period.
+	 * Make sure that a lock is always counted if the corresponding unlock
+	 * is counted. Needs to be a smp_mb() as the read side may contain a
+	 * read from a variable that is written to before the synchronize_srcu()
+	 * in the write side. In this case smp_mb()s A and B act like the store
+	 * buffering pattern.
 	 *
-	 * Also, if the above call to srcu_readers_seq_idx() saw the
-	 * increment of ->seq[], then the call to srcu_readers_active_idx()
-	 * must see the increment of ->c[].
+	 * This smp_mb() also pairs with smp_mb() C to prevent writes after the
+	 * synchronize_srcu() from being executed before the grace period ends.
 	 */
 	smp_mb(); /* A */
 
 	/*
-	 * 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 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;
-
-	/*
-	 * The remainder of this function is the validation step.
-	 * The following smp_mb() D pairs with the smp_mb() C in
-	 * __srcu_read_unlock().  If the __srcu_read_unlock() was seen
-	 * by srcu_readers_active_idx() above, then any destructive
-	 * operation performed after the grace period will happen after
-	 * the corresponding SRCU read-side critical section.
+	 * If the locks are the same as the unlocks, then there must of have
+	 * been no readers on this index at some time in between. This does not
+	 * mean that there are no more readers, as one could have read the
+	 * current index but have incremented the lock counter yet.
 	 *
-	 * Note that there can be at most NR_CPUS worth of readers using
-	 * the old index, which is not enough to overflow even a 32-bit
-	 * integer.  (Yes, this does mean that systems having more than
-	 * a billion or so CPUs need to be 64-bit systems.)  Therefore,
-	 * the sum of the ->seq[] counters cannot possibly overflow.
-	 * Therefore, the only way that the return values of the two
-	 * calls to srcu_readers_seq_idx() can be equal is if there were
-	 * no increments of the corresponding rank of ->seq[] counts
-	 * in the interim.  But the missed-increment scenario laid out
-	 * above includes an increment of the ->seq[] counter by
-	 * the corresponding __srcu_read_lock().  Therefore, if this
-	 * scenario occurs, the return values from the two calls to
-	 * srcu_readers_seq_idx() will differ, and thus the validation
-	 * step below suffices.
+	 * Possible bug: There is no guarantee that there haven't been ULONG_MAX
+	 * increments of ->lock_count[] since the unlocks were counted, meaning
+	 * that this could return true even if there are still active readers.
+	 * Since there are no memory barriers around srcu_flip(), the CPU is not
+	 * required to increment ->completed before running
+	 * srcu_readers_unlock_idx(), which means that there could be an
+	 * arbitrarily large number of critical sections that execute after
+	 * srcu_readers_unlock_idx() but use the old value of ->completed.
 	 */
-	smp_mb(); /* D */
-
-	return srcu_readers_seq_idx(sp, idx) == seq;
+	return srcu_readers_lock_idx(sp, idx) == unlocks;
 }
 

/**

kernel/rcu/srcu.c +9 -6

@@ -266,8 +233,12 @@ static bool srcu_readers_active(struct srcu_struct *sp)
 	unsigned long sum = 0;
 
 	for_each_possible_cpu(cpu) {
-		sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
-		sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
+		struct srcu_struct_array *cpu_counts =
+			per_cpu_ptr(sp->per_cpu_ref, cpu);
+		sum += READ_ONCE(cpu_counts->lock_count[0]);
+		sum += READ_ONCE(cpu_counts->lock_count[1]);
+		sum -= READ_ONCE(cpu_counts->unlock_count[0]);
+		sum -= READ_ONCE(cpu_counts->unlock_count[1]);
 	}
 	return sum;
 }

@@ -298,9 +269,8 @@ int __srcu_read_lock(struct srcu_struct *sp)
 	int idx;
 
 	idx = READ_ONCE(sp->completed) & 0x1;
-	__this_cpu_inc(sp->per_cpu_ref->c[idx]);
+	__this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
 	smp_mb(); /* B */  /* Avoid leaking the critical section. */
-	__this_cpu_inc(sp->per_cpu_ref->seq[idx]);
 	return idx;
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock);

@@ -314,7 +284,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
 void __srcu_read_unlock(struct srcu_struct *sp, int idx)
 {
 	smp_mb(); /* C */  /* Avoid leaking the critical section. */
-	this_cpu_dec(sp->per_cpu_ref->c[idx]);
+	this_cpu_inc(sp->per_cpu_ref->unlock_count[idx]);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 

@@ -349,7 +319,7 @@ static bool try_check_zero(struct srcu_struct *sp, int idx,
 int trycount)
 
 /*
  * Increment the ->completed counter so that future SRCU readers will
- * use the other rank of the ->c[] and ->seq[] arrays.  This allows
+ * use the other rank of the ->(un)lock_count[] arrays.  This allows
  * us to wait for pre-existing readers in a starvation-free manner.
  */

static void srcu_flip(struct srcu_struct *sp)
--
2.9.0

Quoted 24 lines · ----- On Nov 18, 2016, at 9:08 AM, Paul E. McKenney [email protected]…

Quoted 11 lines · Lance Roy

SRCU uses two per-cpu counters: a nesting counter to count the number of active critical sections, and a sequence counter to ensure that the nesting counters don't change while they are being added together in srcu_readers_active_idx_check().

This patch instead uses per-cpu lock and unlock counters. Because the both counters only increase and srcu_readers_active_idx_check() reads the unlock counter before the lock counter, this achieves the same end without having to increment two different counters in srcu_read_lock(). This also saves a smp_mb() in srcu_readers_active_idx_check().

Quoted 4 lines

Possible bug: There is no guarantee that the lock counter won't overflow during srcu_readers_active_idx_check(), as there are no memory barriers around srcu_flip() (see comment in srcu_readers_active_idx_check() for details). However, this problem was already present before this patch.

This patch differs from the previous one in a few (good) code-style changes, comment changes, and of course the commit log. Good. Once discussion converges, I will apply the agreed-upon commit, which might well be this one.

However, let's first take a look at the overflow issue.

Quoted 10 lines

If a given program could have ULONG_MAX or more readers at any given time, there would of course be overflow. However, each read must have an srcu_read_lock() outstanding, and the resulting four-byte return value must be stored somewhere. Because the full address space is at most ULONG_MAX, the maximum number of outstanding readers is at most ULONG_MAX/4, even in the degenerate case where a single CPU/task invokes srcu_read_lock() in a tight loop. And even this assumes that the entire address space can somehow be devoted to srcu_read_lock() return values. ULONG_MAX/4 is therefore a hard upper bound on the number of outstanding SRCU readers.

The loop taking srcu_read_lock() seems a bit far fetched.

More realistically, we could move this bound even lower if we can expect each thread to only nest srcu up to a limited amount (e.g. 128). The realistic limit of number of SRCU readers then becomes bounded by the maximum number of threads multiplied by the srcu max nesting count.

Quoted 29 lines

Now srcu_readers_active_idx_check() checks for strict equality between the number of locks and unlocks, so we can in theory tolerate ULONG_MAX-1 readers. So, the question is whether ULONG_MAX/4 readers can result in the updater seeing ULONG_MAX reads, due to memory misordering and other issues.

Because there are no memory barriers surrounding srcu_flip(), the updater could miss an extremely large number of srcu_read_unlock()s. However, each missed srcu_read_unlock() must have a corresponding srcu_read_lock(), and there is a full memory barrier between between the srcu_flip() and the read of the lock count. There is also a full barrier between any srcu_read_lock()'s increment of the lock count and that CPU's/task's next srcu_read_lock()'s fetch of the index. Therefore, if the updater misses counting a given srcu_read_lock(), that CPU's/task's next srcu_read_lock() must see the new value of the index. Because srcu_read_lock() disables preemption across the index fetch and the lock increment, there can be at most NR_CPUS-1 srcu_read_lock() calls that missed the recent srcu_flip()'s update to the index. (I said NR_CPUS earlier, but Mathieu is correct in pointing out that srcu_flip() has to run somewhere.)

The maximum number of locks that the updater can see is therefore:

o ULONG_MAX/4 for a full set of missed srcu_read_unlock()s.

o ULONG_MAX/4 for a full set of srcu_read_lock()s.

o NR_CPUS-1 for a full set of subsequent srcu_read_lock()s that
missed the flip.

Quoted 6 lines

This totals to ULONG_MAX/2+NR_CPUS-1. So as long as there are no more than ULONG_MAX/2 CPUs, we should be good. And given that the biggest system I have hard evidence of is 4K CPUs, we have ample headrooom compared to the ~2G value of ULONG_MAX/2, even on 32-bit systems.

So, what am I missing here?

Your analysis makes sense.

An alternative approach would be to document some limits on the allowed nesting count for a given SRCU domain that should be expected from a thread, and use that as an even lower upper bound on the number of concurrent SRCU readers, which would allow us to increase the number of supported CPUs accordingly on 32-bit systems.

Thanks,

Mathieu

Quoted 260 lines

Thanx, Paul

Quoted 257 lines

Suggested-by Mathieu Desnoyers <[email protected]>

Signed-off-by Lance Roy <[email protected]>

---
 include/linux/srcu.h    |   4 +-
 kernel/rcu/rcutorture.c |  20 ++++++++-
 kernel/rcu/srcu.c       | 116 ++++++++++++++++++------------------------------
 3 files changed, 63 insertions(+), 77 deletions(-)

include/linux/srcu.h +2 -2

index dc8eb63..0caea34 100644
@@ -34,8 +34,8 @@
 #include <linux/workqueue.h>
 
 struct srcu_struct_array {
-	unsigned long c[2];
-	unsigned long seq[2];
+	unsigned long lock_count[2];
+	unsigned long unlock_count[2];
 };
 
 struct rcu_batch {

kernel/rcu/rcutorture.c +18 -2

index bf08fee..2450c61 100644
@@ -555,10 +555,26 @@ static void srcu_torture_stats(void)
 	pr_alert("%s%s per-CPU(idx=%d):",
 		 torture_type, TORTURE_FLAG, idx);
 	for_each_possible_cpu(cpu) {
+		unsigned long l0, l1;
+		unsigned long u0, u1;
 		long c0, c1;
+		struct srcu_struct_array *counts =
+			per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu);
 
-		c0 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[!idx];
-		c1 = (long)per_cpu_ptr(srcu_ctlp->per_cpu_ref, cpu)->c[idx];
+		u0 = counts->unlock_count[!idx];
+		u1 = counts->unlock_count[idx];
+
+		/*
+		 * Make sure that a lock is always counted if the corresponding
+		 * unlock is counted.
+		 */
+		smp_rmb();
+
+		l0 = counts->lock_count[!idx];
+		l1 = counts->lock_count[idx];
+
+		c0 = (long)(l0 - u0);
+		c1 = (long)(l1 - u1);
 		pr_cont(" %d(%ld,%ld)", cpu, c0, c1);
 	}
 	pr_cont("\n");

kernel/rcu/srcu.c +34 -67

index 9b9cdd5..38e9aae 100644
@@ -141,34 +141,38 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
 /*
- * Returns approximate total of the readers' ->seq[] values for the
+ * Returns approximate total of the readers' ->lock_count[] values for the
  * rank of per-CPU counters specified by idx.
  */
-static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
+static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
 {
 	int cpu;
 	unsigned long sum = 0;
 	unsigned long t;
 
 	for_each_possible_cpu(cpu) {
-		t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
+		struct srcu_struct_array *cpu_counts =
+			per_cpu_ptr(sp->per_cpu_ref, cpu);
+		t = READ_ONCE(cpu_counts->lock_count[idx]);
 		sum += t;
 	}
 	return sum;
 }
 
 /*
- * Returns approximate number of readers active on the specified rank
- * of the per-CPU ->c[] counters.
+ * Returns approximate total of the readers' ->unlock_count[] values for the
+ * rank of per-CPU counters specified by idx.
  */
-static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
+static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
 {
 	int cpu;
 	unsigned long sum = 0;
 	unsigned long t;
 
 	for_each_possible_cpu(cpu) {
-		t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
+		struct srcu_struct_array *cpu_counts =
+			per_cpu_ptr(sp->per_cpu_ref, cpu);
+		t = READ_ONCE(cpu_counts->unlock_count[idx]);
 		sum += t;
 	}
 	return sum;

@@ -176,79 +180,42 @@ static unsigned long srcu_readers_active_idx(struct
 srcu_struct *sp, int idx)
 
 /*
  * Return true if the number of pre-existing readers is determined to
- * be stably zero.  An example unstable zero can occur if the call
- * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
- * but due to task migration, sees the corresponding __srcu_read_unlock()
- * decrement.  This can happen because srcu_readers_active_idx() takes
- * time to sum the array, and might in fact be interrupted or preempted
- * partway through the summation.
+ * be zero.
  */
 static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
 {
-	unsigned long seq;
+	unsigned long unlocks;
 
-	seq = srcu_readers_seq_idx(sp, idx);
+	unlocks = srcu_readers_unlock_idx(sp, idx);
 
 	/*
-	 * The following smp_mb() A pairs with the smp_mb() B located in
-	 * __srcu_read_lock().  This pairing ensures that if an
-	 * __srcu_read_lock() increments its counter after the summation
-	 * in srcu_readers_active_idx(), then the corresponding SRCU read-side
-	 * critical section will see any changes made prior to the start
-	 * of the current SRCU grace period.
+	 * Make sure that a lock is always counted if the corresponding unlock
+	 * is counted. Needs to be a smp_mb() as the read side may contain a
+	 * read from a variable that is written to before the synchronize_srcu()
+	 * in the write side. In this case smp_mb()s A and B act like the store
+	 * buffering pattern.
 	 *
-	 * Also, if the above call to srcu_readers_seq_idx() saw the
-	 * increment of ->seq[], then the call to srcu_readers_active_idx()
-	 * must see the increment of ->c[].
+	 * This smp_mb() also pairs with smp_mb() C to prevent writes after the
+	 * synchronize_srcu() from being executed before the grace period ends.
 	 */
 	smp_mb(); /* A */
 
 	/*
-	 * 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 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;
-
-	/*
-	 * The remainder of this function is the validation step.
-	 * The following smp_mb() D pairs with the smp_mb() C in
-	 * __srcu_read_unlock().  If the __srcu_read_unlock() was seen
-	 * by srcu_readers_active_idx() above, then any destructive
-	 * operation performed after the grace period will happen after
-	 * the corresponding SRCU read-side critical section.
+	 * If the locks are the same as the unlocks, then there must of have
+	 * been no readers on this index at some time in between. This does not
+	 * mean that there are no more readers, as one could have read the
+	 * current index but have incremented the lock counter yet.
 	 *
-	 * Note that there can be at most NR_CPUS worth of readers using
-	 * the old index, which is not enough to overflow even a 32-bit
-	 * integer.  (Yes, this does mean that systems having more than
-	 * a billion or so CPUs need to be 64-bit systems.)  Therefore,
-	 * the sum of the ->seq[] counters cannot possibly overflow.
-	 * Therefore, the only way that the return values of the two
-	 * calls to srcu_readers_seq_idx() can be equal is if there were
-	 * no increments of the corresponding rank of ->seq[] counts
-	 * in the interim.  But the missed-increment scenario laid out
-	 * above includes an increment of the ->seq[] counter by
-	 * the corresponding __srcu_read_lock().  Therefore, if this
-	 * scenario occurs, the return values from the two calls to
-	 * srcu_readers_seq_idx() will differ, and thus the validation
-	 * step below suffices.
+	 * Possible bug: There is no guarantee that there haven't been ULONG_MAX
+	 * increments of ->lock_count[] since the unlocks were counted, meaning
+	 * that this could return true even if there are still active readers.
+	 * Since there are no memory barriers around srcu_flip(), the CPU is not
+	 * required to increment ->completed before running
+	 * srcu_readers_unlock_idx(), which means that there could be an
+	 * arbitrarily large number of critical sections that execute after
+	 * srcu_readers_unlock_idx() but use the old value of ->completed.
 	 */
-	smp_mb(); /* D */
-
-	return srcu_readers_seq_idx(sp, idx) == seq;
+	return srcu_readers_lock_idx(sp, idx) == unlocks;
 }
 

/**

kernel/rcu/srcu.c +9 -6

@@ -266,8 +233,12 @@ static bool srcu_readers_active(struct srcu_struct *sp)
 	unsigned long sum = 0;
 
 	for_each_possible_cpu(cpu) {
-		sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
-		sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
+		struct srcu_struct_array *cpu_counts =
+			per_cpu_ptr(sp->per_cpu_ref, cpu);
+		sum += READ_ONCE(cpu_counts->lock_count[0]);
+		sum += READ_ONCE(cpu_counts->lock_count[1]);
+		sum -= READ_ONCE(cpu_counts->unlock_count[0]);
+		sum -= READ_ONCE(cpu_counts->unlock_count[1]);
 	}
 	return sum;
 }

@@ -298,9 +269,8 @@ int __srcu_read_lock(struct srcu_struct *sp)
 	int idx;
 
 	idx = READ_ONCE(sp->completed) & 0x1;
-	__this_cpu_inc(sp->per_cpu_ref->c[idx]);
+	__this_cpu_inc(sp->per_cpu_ref->lock_count[idx]);
 	smp_mb(); /* B */  /* Avoid leaking the critical section. */
-	__this_cpu_inc(sp->per_cpu_ref->seq[idx]);
 	return idx;
 }
 EXPORT_SYMBOL_GPL(__srcu_read_lock);

@@ -314,7 +284,7 @@ EXPORT_SYMBOL_GPL(__srcu_read_lock);
 void __srcu_read_unlock(struct srcu_struct *sp, int idx)
 {
 	smp_mb(); /* C */  /* Avoid leaking the critical section. */
-	this_cpu_dec(sp->per_cpu_ref->c[idx]);
+	this_cpu_inc(sp->per_cpu_ref->unlock_count[idx]);
 }
 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
 

@@ -349,7 +319,7 @@ static bool try_check_zero(struct srcu_struct *sp, int idx,
 int trycount)
 
 /*
  * Increment the ->completed counter so that future SRCU readers will
- * use the other rank of the ->c[] and ->seq[] arrays.  This allows
+ * use the other rank of the ->(un)lock_count[] arrays.  This allows
  * us to wait for pre-existing readers in a starvation-free manner.
  */

static void srcu_flip(struct srcu_struct *sp)
--
2.9.0

Signature · 3 lines

Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

However, let's first take a look at the overflow issue.

If a given program could have ULONG_MAX or more readers at any given time, there would of course be overflow. However, each read must have an srcu_read_lock() outstanding, and the resulting four-byte return value must be stored somewhere. Because the full address space is at most ULONG_MAX, the maximum number of outstanding readers is at most ULONG_MAX/4, even in the degenerate case where a single CPU/task invokes srcu_read_lock() in a tight loop. And even this assumes that the entire address space can somehow be devoted to srcu_read_lock() return values. ULONG_MAX/4 is therefore a hard upper bound on the number of outstanding SRCU readers.

Now srcu_readers_active_idx_check() checks for strict equality between the number of locks and unlocks, so we can in theory tolerate ULONG_MAX-1 readers. So, the question is whether ULONG_MAX/4 readers can result in the updater seeing ULONG_MAX reads, due to memory misordering and other issues.

Because there are no memory barriers surrounding srcu_flip(), the updater could miss an extremely large number of srcu_read_unlock()s. However, each missed srcu_read_unlock() must have a corresponding srcu_read_lock(), and there is a full memory barrier between between the srcu_flip() and the read of the lock count. There is also a full barrier between any srcu_read_lock()'s increment of the lock count and that CPU's/task's next srcu_read_lock()'s fetch of the index. Therefore, if the updater misses counting a given srcu_read_lock(), that CPU's/task's next srcu_read_lock() must see the new value of the index. Because srcu_read_lock() disables preemption across the index fetch and the lock increment, there can be at most NR_CPUS-1 srcu_read_lock() calls that missed the recent srcu_flip()'s update to the index. (I said NR_CPUS earlier, but Mathieu is correct in pointing out that srcu_flip() has to run somewhere.)

Quoted 12 lines · On Fri, 18 Nov 2016 06:08:45 -0800 "Paul E. McKenney"…

However, let's first take a look at the overflow issue.

If a given program could have ULONG_MAX or more readers at any given time, there would of course be overflow. However, each read must have an srcu_read_lock() outstanding, and the resulting four-byte return value must be stored somewhere. Because the full address space is at most ULONG_MAX, the maximum number of outstanding readers is at most ULONG_MAX/4, even in the degenerate case where a single CPU/task invokes srcu_read_lock() in a tight loop. And even this assumes that the entire address space can somehow be devoted to srcu_read_lock() return values. ULONG_MAX/4 is therefore a hard upper bound on the number of outstanding SRCU readers.

I agree that there should be at most ULONG_MAX/4 active readers at a time, as long as the compiler doesn't do something crazy like noticing that srcu_read_lock() always returns 0 or 1 and then packing the return values into smaller variables.

Quoted 6 lines

Now srcu_readers_active_idx_check() checks for strict equality between the number of locks and unlocks, so we can in theory tolerate ULONG_MAX-1 readers. So, the question is whether ULONG_MAX/4 readers can result in the updater seeing ULONG_MAX reads, due to memory misordering and other issues.

Quoted 13 lines

Because there are no memory barriers surrounding srcu_flip(), the updater could miss an extremely large number of srcu_read_unlock()s. However, each missed srcu_read_unlock() must have a corresponding srcu_read_lock(), and there is a full memory barrier between between the srcu_flip() and the read of the lock count. There is also a full barrier between any srcu_read_lock()'s increment of the lock count and that CPU's/task's next srcu_read_lock()'s fetch of the index. Therefore, if the updater misses counting a given srcu_read_lock(), that CPU's/task's next srcu_read_lock() must see the new value of the index. Because srcu_read_lock() disables preemption across the index fetch and the lock increment, there can be at most NR_CPUS-1 srcu_read_lock() calls that missed the recent srcu_flip()'s update to the index. (I said NR_CPUS earlier, but Mathieu is correct in pointing out that srcu_flip() has to run somewhere.)

The trouble is that disabling preemption is not enough to ensure that there is at most one srcu_read_lock() call per CPU that missed the srcu_flip().

Define a reader to be an SRCU lock+unlock pair. A reader is called active if it has incremented ->lock_count[] but hasn't incremented ->unlock_count[] yet, and completed if it has incremented ->unlock_count[]. I think that we only want to limit the number of active readers and the number of CPUs. In particular, I don't think there should be a limit on the rate of completion of read side critical section.

The goal of srcu_readers_active_idx_check() is to verify that there were zero active readers on the inactive index at some time during its execution. To do this, it totals the unlock counts, executes a memory barrier, totals the lock counts, and takes the difference. This difference counts the readers that are active when srcu_readers_lock_idx() gets to their CPU, plus the readers that completed after srcu_readers_unlock_idx() and before srcu_readers_lock_idx(). If the true (infinite precision) value of the difference is zero, then there were no active readers at some point while srcu_readers_lock_idx() is running. However, the difference is only stored in a long, so there is a potential for overflow if too many readers complete during srcu_readers_active_idx_check().

For example, let's say there are three threads, each running on their own CPU:

int data, flag;
struct srcu_struct *sp = /* ... */;

Thread 0:

	data = 1;
	synchronize_srcu(sp);
	flag = 1;

Thread 1:

	int data_r, flag_r;
	int idx = srcu_read_lock(sp);
	data_r = data;
	flag_r = flag;
	srcu_read_unlock(sp, idx);
	BUG_ON(flag_r == 1 && data_r == 0);

Thread 2:

	while (true) {
		int idx = srcu_read_lock(sp);
		srcu_read_unlock(sp, idx);
	}

Let's take the following execution order. Thread 1 increments
the CPU 1 version of sp->lock_count[0], sets idx to zero, and loads data (0) into data_r. Thread 0 then sets data to be 1, verifies that there are no readers on index 1, and increments sp->completed, but the CPU actually doesn't preform the last operation, putting it off until the next memory barrier. Thread 0 then calls srcu_readers_active_idx_check() on index 0, which runs srcu_readers_unlock_idx() (returning 0). Right after srcu_readers_unlock_idx() completes, thread 2 starts running. Since Thread 0 hasn't actually incremented sp->completed in any way that is visible to thread 2, srcu_read_lock() will still return 0. Thread 2 can then run for ULONG_MAX iterations, setting the CPU 2 version of sp->unlock_count[0] to ULONG_MAX. CPU 0 then finally gets around to incrementing sp->completed, runs its memory barrier, and then reads the lock counts: 1, 0, ULONG_MAX. The total of ULONG_MAX+1 will overflow to 0 and compare equal with earlier unlock count. Thread 0 will then think that the grace period is over and set flag to one. Thread 1 can then read flag (1) into flag_r and run srcu_read_unlock(). The BUG_ON statement will then fail.

Although ULONG_MAX readers completed during srcu_readers_active_idx_check(), there were at most 2 active readers at any time, so this program doesn't run into any limit.

I hope that was clear enough.

Quoted 69 lines · Lance Roy

The trouble is that disabling preemption is not enough to ensure that there is at most one srcu_read_lock() call per CPU that missed the srcu_flip().

Define a reader to be an SRCU lock+unlock pair. A reader is called active if it has incremented ->lock_count[] but hasn't incremented ->unlock_count[] yet, and completed if it has incremented ->unlock_count[]. I think that we only want to limit the number of active readers and the number of CPUs. In particular, I don't think there should be a limit on the rate of completion of read side critical section.

The goal of srcu_readers_active_idx_check() is to verify that there were zero active readers on the inactive index at some time during its execution. To do this, it totals the unlock counts, executes a memory barrier, totals the lock counts, and takes the difference. This difference counts the readers that are active when srcu_readers_lock_idx() gets to their CPU, plus the readers that completed after srcu_readers_unlock_idx() and before srcu_readers_lock_idx(). If the true (infinite precision) value of the difference is zero, then there were no active readers at some point while srcu_readers_lock_idx() is running. However, the difference is only stored in a long, so there is a potential for overflow if too many readers complete during srcu_readers_active_idx_check().

For example, let's say there are three threads, each running on their own CPU:

int data, flag;
struct srcu_struct *sp = /* ... */;

Thread 0:

	data = 1;
	synchronize_srcu(sp);
	flag = 1;

Thread 1:

	int data_r, flag_r;
	int idx = srcu_read_lock(sp);
	data_r = data;
	flag_r = flag;
	srcu_read_unlock(sp, idx);
	BUG_ON(flag_r == 1 && data_r == 0);

Thread 2:

	while (true) {
		int idx = srcu_read_lock(sp);
		srcu_read_unlock(sp, idx);
	}

Let's take the following execution order. Thread 1 increments the CPU 1 version of sp->lock_count[0], sets idx to zero, and loads data (0) into data_r. Thread 0 then sets data to be 1, verifies that there are no readers on index 1, and increments sp->completed, but the CPU actually doesn't preform the last operation, putting it off until the next memory barrier. Thread 0 then calls srcu_readers_active_idx_check() on index 0, which runs srcu_readers_unlock_idx() (returning 0). Right after srcu_readers_unlock_idx() completes, thread 2 starts running. Since Thread 0 hasn't actually incremented sp->completed in any way that is visible to thread 2, srcu_read_lock() will still return 0. Thread 2 can then run for ULONG_MAX iterations, setting the CPU 2 version of sp->unlock_count[0] to ULONG_MAX. CPU 0 then finally gets around to incrementing sp->completed, runs its memory barrier, and then reads the lock counts: 1, 0, ULONG_MAX. The total of ULONG_MAX+1 will overflow to 0 and compare equal with earlier unlock count. Thread 0 will then think that the grace period is over and set flag to one. Thread 1 can then read flag (1) into flag_r and run srcu_read_unlock(). The BUG_ON statement will then fail.

Although ULONG_MAX readers completed during srcu_readers_active_idx_check(), there were at most 2 active readers at any time, so this program doesn't run into any limit.

Quoted 1 line

I hope that was clear enough.

Indeed it is!

So adding a full memory barrier immediately after the srcu_flip() should prevent this, because if the updater failed to see an unlock increment, the second following lock for that CPU/task would be guaranteed to see the flip. Or am I still missing something?

Is there a sequence of events that requires a full memory barrier before the srcu_flip()?

Quoted 70 lines · On Fri, 18 Nov 2016 15:13:45 -0800 "Paul E. McKenney"…

Quoted 69 lines · Lance Roy

The trouble is that disabling preemption is not enough to ensure that there is at most one srcu_read_lock() call per CPU that missed the srcu_flip().

Define a reader to be an SRCU lock+unlock pair. A reader is called active if it has incremented ->lock_count[] but hasn't incremented ->unlock_count[] yet, and completed if it has incremented ->unlock_count[]. I think that we only want to limit the number of active readers and the number of CPUs. In particular, I don't think there should be a limit on the rate of completion of read side critical section.

The goal of srcu_readers_active_idx_check() is to verify that there were zero active readers on the inactive index at some time during its execution. To do this, it totals the unlock counts, executes a memory barrier, totals the lock counts, and takes the difference. This difference counts the readers that are active when srcu_readers_lock_idx() gets to their CPU, plus the readers that completed after srcu_readers_unlock_idx() and before srcu_readers_lock_idx(). If the true (infinite precision) value of the difference is zero, then there were no active readers at some point while srcu_readers_lock_idx() is running. However, the difference is only stored in a long, so there is a potential for overflow if too many readers complete during srcu_readers_active_idx_check().

For example, let's say there are three threads, each running on their own CPU:

int data, flag;
struct srcu_struct *sp = /* ... */;

Thread 0:

	data = 1;
	synchronize_srcu(sp);
	flag = 1;

Thread 1:

	int data_r, flag_r;
	int idx = srcu_read_lock(sp);
	data_r = data;
	flag_r = flag;
	srcu_read_unlock(sp, idx);
	BUG_ON(flag_r == 1 && data_r == 0);

Thread 2:

	while (true) {
		int idx = srcu_read_lock(sp);
		srcu_read_unlock(sp, idx);
	}

Let's take the following execution order. Thread 1 increments the CPU 1 version of sp->lock_count[0], sets idx to zero, and loads data (0) into data_r. Thread 0 then sets data to be 1, verifies that there are no readers on index 1, and increments sp->completed, but the CPU actually doesn't preform the last operation, putting it off until the next memory barrier. Thread 0 then calls srcu_readers_active_idx_check() on index 0, which runs srcu_readers_unlock_idx() (returning 0). Right after srcu_readers_unlock_idx() completes, thread 2 starts running. Since Thread 0 hasn't actually incremented sp->completed in any way that is visible to thread 2, srcu_read_lock() will still return 0. Thread 2 can then run for ULONG_MAX iterations, setting the CPU 2 version of sp->unlock_count[0] to ULONG_MAX. CPU 0 then finally gets around to incrementing sp->completed, runs its memory barrier, and then reads the lock counts: 1, 0, ULONG_MAX. The total of ULONG_MAX+1 will overflow to 0 and compare equal with earlier unlock count. Thread 0 will then think that the grace period is over and set flag to one. Thread 1 can then read flag (1) into flag_r and run srcu_read_unlock(). The BUG_ON statement will then fail.

Although ULONG_MAX readers completed during srcu_readers_active_idx_check(), there were at most 2 active readers at any time, so this program doesn't run into any limit.

Quoted 8 lines

Quoted 1 line

I hope that was clear enough.

Indeed it is!

So adding a full memory barrier immediately after the srcu_flip() should prevent this, because if the updater failed to see an unlock increment, the second following lock for that CPU/task would be guaranteed to see the flip. Or am I still missing something?

Yes, adding a full memory barrier after srcu_flip() prevents this problem.

Quoted 2 lines

Is there a sequence of events that requires a full memory barrier before the srcu_flip()?

I am now unsure if a memory barrier before srcu_flip() is necessary. I thought that it would be needed to prevent the CPU from preforming the increment early, but I have just noticed that srcu_advance_batches() will return early if the first try_check_zero() fails, creating a control dependency. I think that this control dependency should be enough to prevent the problem from occurring.

One interesting thing about this version is that there is only an address dependency between the load of ->completed and the increment of ->load_count[]. This means that without an smp_read_barrier_depends() between the two, a reader could use the new value of ->completed to increment ->load_count[], before actually reading ->completed. (I was surprised that this ordering doesn't come for free, as it seems like violating it would require speculative writes.) It doesn't matter much if the dependency barrier is actually there, because leaving it out just means at most 1 more spurious increment per CPU.

I think that with the added synchronization in srcu_flip() there would be at most 3 * NR_CPUS readers that start and complete inside one srcu_readers_active_idx_check(). With the read side dependency barrier this would drop to 2 * NR_CPUS.