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[PATCH tip/core/rcu 1/3] srcu: More efficient reader counts.

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  [PATCH tip/core/rcu 1/3] srcu: More efficient reader counts. "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:30 +0100
    Re: [PATCH tip/core/rcu 1/3] srcu: More efficient reader counts. Ingo Molnar <mingo@kernel.org> - 2017-01-14 10:40 +0100
      Re: [PATCH tip/core/rcu 1/3] srcu: More efficient reader counts. "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-15 04:50 +0100

#1558913 — [PATCH tip/core/rcu 1/3] srcu: More efficient reader counts.

From: Lance Roy <ldr709@gmail.com>

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().

A possible problem with this patch is that it can only handle
ULONG_MAX - NR_CPUS simultaneous readers, whereas the old version could
handle up to ULONG_MAX.

Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Lance Roy <ldr709@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
---
 include/linux/srcu.h    |   4 +-
 kernel/rcu/rcutorture.c |  18 +++++++-
 kernel/rcu/srcu.c       | 117 ++++++++++++++++++------------------------------
 3 files changed, 62 insertions(+), 77 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index dc8eb63c6568..0caea34d8c5f 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -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 {
diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 87c51225ceec..6e4fd7680c70 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -564,10 +564,24 @@ 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");
diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index 9b9cdd549caa..edfdfadec821 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -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 +180,43 @@ 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.
+	 * Note that there can be at most NR_CPUS worth of readers using the old
+	 * index that haven't incremented ->lock_count[] yet.  Therefore, the
+	 * sum of the ->lock_count[]s cannot increment enough times to overflow
+	 * and end up equal the sum of the ->unlock_count[]s, as long as there
+	 * are at most ULONG_MAX - NR_CPUS readers at a time.  (Yes, this does
+	 * mean that systems having more than a billion or so CPUs need to be
+	 * 64-bit systems.)  Therefore, the only way that the return values of
+	 * the two calls to srcu_readers_(un)lock_idx() can be equal is if there
+	 * are no active readers using this index.
 	 */
-	smp_mb(); /* D */
-
-	return srcu_readers_seq_idx(sp, idx) == seq;
+	return srcu_readers_lock_idx(sp, idx) == unlocks;
 }
 
 /**
@@ -266,8 +234,12 @@ 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 +270,8 @@ 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 +285,7 @@ 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 +320,7 @@ 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)
-- 
2.5.2

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#1558921

Noticed a few minor nits:

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> From: Lance Roy <ldr709@gmail.com>
> 
> 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().

typo:

 s/Because the both counters
   Because both counters

> 
> A possible problem with this patch is that it can only handle
> ULONG_MAX - NR_CPUS simultaneous readers, whereas the old version could
> handle up to ULONG_MAX.

I don't think this is a problem! :-)

> 
> Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Lance Roy <ldr709@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
>  include/linux/srcu.h    |   4 +-
>  kernel/rcu/rcutorture.c |  18 +++++++-
>  kernel/rcu/srcu.c       | 117 ++++++++++++++++++------------------------------
>  3 files changed, 62 insertions(+), 77 deletions(-)
> 
> diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> index dc8eb63c6568..0caea34d8c5f 100644
> --- a/include/linux/srcu.h
> +++ b/include/linux/srcu.h
> @@ -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 {
> diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> index 87c51225ceec..6e4fd7680c70 100644
> --- a/kernel/rcu/rcutorture.c
> +++ b/kernel/rcu/rcutorture.c
> @@ -564,10 +564,24 @@ 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);

Please don't break the line to pacify checkpatch - if the line is too long then 
maybe split out the loop body into a helper function - but keeping it a bit longer 
than 80 cols is fine as well.

>  
> -		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();

That's not the standard kernel code comment style.

> +
> +		l0 = counts->lock_count[!idx];
> +		l1 = counts->lock_count[idx];
> +
> +		c0 = (long)(l0 - u0);
> +		c1 = (long)(l1 - u1);

These type casts look unnecessary to me.

>  	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;


>  	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;

These linebreak look ugly as well. Some abbreviation of types and variables might 
help:

	s/srcu_struct_array/srcu_array
	s/cpu_counts/cpuc

?

> +	 * 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 that haven't incremented ->lock_count[] yet.  Therefore, the
> +	 * sum of the ->lock_count[]s cannot increment enough times to overflow
> +	 * and end up equal the sum of the ->unlock_count[]s, as long as there
> +	 * are at most ULONG_MAX - NR_CPUS readers at a time.  (Yes, this does
> +	 * mean that systems having more than a billion or so CPUs need to be
> +	 * 64-bit systems.)  Therefore, the only way that the return values of
> +	 * the two calls to srcu_readers_(un)lock_idx() can be equal is if there
> +	 * are no active readers using this index.

typo:

   s/must of have been no readers/
     must have been no readers

Also, maybe I'm misreading it, but shouldn't it be:

   s/as one could have read the current index but have incremented the lock counter yet.
    /as one could have read the current index but not have incremented the lock counter yet.

?


Also, the title:

   srcu: More efficient reader counts.

should have a verb and no full stop, i.e. something like:

   srcu: Implement more efficient reader counts

Thanks,

	Ingo

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#1559149

On Sat, Jan 14, 2017 at 10:31:15AM +0100, Ingo Molnar wrote:
> 
> Noticed a few minor nits:

And thank you for the review and comments!

> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > From: Lance Roy <ldr709@gmail.com>
> > 
> > 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().
> 
> typo:
> 
>  s/Because the both counters
>    Because both counters

Fixed!

> > A possible problem with this patch is that it can only handle
> > ULONG_MAX - NR_CPUS simultaneous readers, whereas the old version could
> > handle up to ULONG_MAX.
> 
> I don't think this is a problem! :-)

Here is hoping!  ;-)

> > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Signed-off-by: Lance Roy <ldr709@gmail.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > ---
> >  include/linux/srcu.h    |   4 +-
> >  kernel/rcu/rcutorture.c |  18 +++++++-
> >  kernel/rcu/srcu.c       | 117 ++++++++++++++++++------------------------------
> >  3 files changed, 62 insertions(+), 77 deletions(-)
> > 
> > diff --git a/include/linux/srcu.h b/include/linux/srcu.h
> > index dc8eb63c6568..0caea34d8c5f 100644
> > --- a/include/linux/srcu.h
> > +++ b/include/linux/srcu.h
> > @@ -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 {
> > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > index 87c51225ceec..6e4fd7680c70 100644
> > --- a/kernel/rcu/rcutorture.c
> > +++ b/kernel/rcu/rcutorture.c
> > @@ -564,10 +564,24 @@ 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);
> 
> Please don't break the line to pacify checkpatch - if the line is too long then 
> maybe split out the loop body into a helper function - but keeping it a bit longer 
> than 80 cols is fine as well.

Creating a helper function woujld leave me several characters over still,
so I just created the long line.  Another approach would be to split the
definition and the initialization into two statements, but that would
add a line.

> > -		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();
> 
> That's not the standard kernel code comment style.

That is embarrassing!  Fixed.

> > +
> > +		l0 = counts->lock_count[!idx];
> > +		l1 = counts->lock_count[idx];
> > +
> > +		c0 = (long)(l0 - u0);
> > +		c1 = (long)(l1 - u1);
> 
> These type casts look unnecessary to me.

Indeed, given that we are assigning to a long rather than just computing.

> >  	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;
> 
> 
> >  	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;
> 
> These linebreak look ugly as well. Some abbreviation of types and variables might 
> help:
> 
> 	s/srcu_struct_array/srcu_array
> 	s/cpu_counts/cpuc
> 
> ?

Why not?  Fixed.  ;-)

> > +	 * 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 that haven't incremented ->lock_count[] yet.  Therefore, the
> > +	 * sum of the ->lock_count[]s cannot increment enough times to overflow
> > +	 * and end up equal the sum of the ->unlock_count[]s, as long as there
> > +	 * are at most ULONG_MAX - NR_CPUS readers at a time.  (Yes, this does
> > +	 * mean that systems having more than a billion or so CPUs need to be
> > +	 * 64-bit systems.)  Therefore, the only way that the return values of
> > +	 * the two calls to srcu_readers_(un)lock_idx() can be equal is if there
> > +	 * are no active readers using this index.
> 
> typo:
> 
>    s/must of have been no readers/
>      must have been no readers
> 
> Also, maybe I'm misreading it, but shouldn't it be:
> 
>    s/as one could have read the current index but have incremented the lock counter yet.
>     /as one could have read the current index but not have incremented the lock counter yet.
> 
> ?

Agreed on both, fixed.

> Also, the title:
> 
>    srcu: More efficient reader counts.
> 
> should have a verb and no full stop, i.e. something like:
> 
>    srcu: Implement more efficient reader counts

And this one as well.

								Thanx, Paul

> Thanks,
> 
> 	Ingo
> 

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