Message-ID: <20061119214315.GI4427@us.ibm.com>
Date:	Sun, 19 Nov 2006 13:43:15 -0800
From:	"Paul E. McKenney" <paulmck@...ibm.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

On Sat, Nov 18, 2006 at 04:00:35PM -0500, Alan Stern wrote:
> On Sat, 18 Nov 2006, Paul E. McKenney wrote:
> 
> > > > @@ -94,7 +112,8 @@ void cleanup_srcu_struct(struct srcu_str
> > > >  	WARN_ON(sum);  /* Leakage unless caller handles error. */
> > > >  	if (sum != 0)
> > > >  		return;
> > > > -	free_percpu(sp->per_cpu_ref);
> > > > +	if (sp->per_cpu_ref != NULL)
> > > > +		free_percpu(sp->per_cpu_ref);
> > >
> > > Now that Andrew has accepted the "allow free_percpu(NULL)" change, you can
> > > remove the test here.
> >
> > OK.  I thought that there was some sort of error-checking involved,
> > but if not, will fix.
> 
> Just make sure that _you_ have the free_percpu(NULL) patch installed on
> your machine before testing this -- otherwise you'll get a nice hard
> crash!

'Nuff said -- will leave this fixup till later.  ;-)

> > > >  	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. */
> > > > +	sap = rcu_dereference(sp->per_cpu_ref);
> > > > +	if (likely(sap != NULL)) {
> > > > +		barrier();  /* ensure compiler looks -once- at sp->completed. */
> > >
> > > Put this barrier() back where the old one was (outside the "if").
> >
> > Why?  Outside this "if", I don't use "sap".
> 
> Because it looks funny to see the comment here talking about sp->completed
> when sp->completed hasn't been used for several lines.  (Maybe it looks
> less funny in the patched source than in the patch itself.)  The best
> place to prevent extra accesses of sp->completed is immediately after
> the required access.

Good point -- and with the addition of the second element of hardluckref,
it has to be hoisted out of the "if" in any case.

> > > > +			    smp_processor_id())->c[idx]++;
> > > > +		smp_mb();
> > > > +		preempt_enable();
> > > > +		return idx;
> > > > +	}
> > > > +	if (mutex_trylock(&sp->mutex)) {
> > > > +		preempt_enable();
> > >
> > > Move the preempt_enable() before the "if", then get rid of the
> > > preempt_enable() after the "if" block.
> >
> > No can do.  The preempt_enable() must follow the increment and
> > the memory barrier, otherwise the synchronize_sched() inside
> > synchronize_srcu() can't do its job.
> 
> You misunderstood -- I was talking about the preempt_enable() in the last
> line quoted above (not the one in the third line) and the "if
> (mutex_trylock" (not the earlier "if (likely").

OK, I see your point -- but this has changed thoroughly with the
addition of the second element of hardluckref.

> > > > +		if (sp->per_cpu_ref == NULL)
> > > > +			sp->per_cpu_ref = alloc_srcu_struct_percpu();
> > >
> > > It would be cleaner to put the mutex_unlock() and closing '}' right here.
> >
> > I can move the mutex_unlock() to this point, but I cannot otherwise
> > merge the two following pieces of code -- at least not without doing
> > an otherwise-gratuitous preempt_disable().  Which I suppose I could
> > do, but seems like it would be more confusing than would the
> > separate code.  I will play with this a bit and see if I can eliminate
> > the duplication.
> 
> If you follow the advice above then you won't need to add a gratuitous
> preempt_disable().  Try it and see how it comes out; the idea is that
> you can use the same code for testing sp->per_cpu_ref regardless of
> whether the mutex_trylock() or the call to alloc_srcu_struct_percpu()
> succeeded.

Understood, finally -- but the two-element hardluckref now requires
greater preempt_disable() coverage.

> > > What happens if a prior reader failed to allocate the memory but this call
> > > succeeds?  You need to check hardluckref before doing this.  The same is
> > > true in srcu_read_lock().
> >
> > All accounted for by the fact that hardluckref is unconditionally
> > added in by srcu_readers_active().  Right?
> 
> Yes, you're right.
> 
> > Will spin a new patch...
> 
> Good -- it's getting pretty messy to look at this one!
> 
> By the way, I think the fastpath for synchronize_srcu() should be safe,
> now that you have added the memory barriers into srcu_read_lock() and
> srcu_read_unlock().  You might as well try putting it in.
> 
> Although now that I look at it again, you have forgotten to put smp_mb()
> after the atomic_inc() call and before the atomic_dec().  In
> srcu_read_unlock() you could just move the existing smp_mb() back before
> the test of idx.

Good catch again -- added smp_mb__before_atomic_dec() and
smp_mb__after_atomic_inc().  The reason for avoiding moving the smp_mb()
is that atomic_dec() implies a memory barrier on some architectures,
such as x86.  In these cases, smp_mb__before_atomic_dec() is a no-op.

Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> (was @us.ibm.com)
---

 include/linux/srcu.h |    8 ---
 kernel/srcu.c        |  126 +++++++++++++++++++++++++++------------------------
 2 files changed, 71 insertions(+), 63 deletions(-)

diff -urpNa -X dontdiff linux-2.6.19-rc5/include/linux/srcu.h linux-2.6.19-rc5-dsrcu/include/linux/srcu.h
--- linux-2.6.19-rc5/include/linux/srcu.h	2006-11-17 15:44:40.000000000 -0800
+++ linux-2.6.19-rc5-dsrcu/include/linux/srcu.h	2006-11-19 13:33:35.000000000 -0800
@@ -35,19 +35,15 @@ struct srcu_struct {
 	int completed;
 	struct srcu_struct_array *per_cpu_ref;
 	struct mutex mutex;
+	atomic_t hardluckref[2];
 };
 
-#ifndef CONFIG_PREEMPT
-#define srcu_barrier() barrier()
-#else /* #ifndef CONFIG_PREEMPT */
-#define srcu_barrier()
-#endif /* #else #ifndef CONFIG_PREEMPT */
-
 int init_srcu_struct(struct srcu_struct *sp);
 void cleanup_srcu_struct(struct srcu_struct *sp);
 int srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
 void srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
 void synchronize_srcu(struct srcu_struct *sp);
 long srcu_batches_completed(struct srcu_struct *sp);
+int srcu_readers_active(struct srcu_struct *sp);
 
 #endif
diff -urpNa -X dontdiff linux-2.6.19-rc5/kernel/srcu.c linux-2.6.19-rc5-dsrcu/kernel/srcu.c
--- linux-2.6.19-rc5/kernel/srcu.c	2006-11-17 15:44:40.000000000 -0800
+++ linux-2.6.19-rc5-dsrcu/kernel/srcu.c	2006-11-19 13:40:33.000000000 -0800
@@ -34,6 +34,18 @@
 #include <linux/smp.h>
 #include <linux/srcu.h>
 
+/*
+ * Initialize the per-CPU array, returning the pointer.
+ */
+static inline struct srcu_struct_array *alloc_srcu_struct_percpu(void)
+{
+	struct srcu_struct_array *sap;
+
+	sap = alloc_percpu(struct srcu_struct_array);
+	smp_wmb();
+	return (sap);
+}
+
 /**
  * init_srcu_struct - initialize a sleep-RCU structure
  * @sp: structure to initialize.
@@ -46,7 +58,9 @@ int init_srcu_struct(struct srcu_struct 
 {
 	sp->completed = 0;
 	mutex_init(&sp->mutex);
-	sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
+	sp->per_cpu_ref = alloc_srcu_struct_percpu();
+	atomic_set(&sp->hardluckref[0], 0);
+	atomic_set(&sp->hardluckref[1], 0);
 	return (sp->per_cpu_ref ? 0 : -ENOMEM);
 }
 
@@ -58,12 +72,15 @@ int init_srcu_struct(struct srcu_struct 
 static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
 {
 	int cpu;
+	struct srcu_struct_array *sap;
 	int sum;
 
 	sum = 0;
-	for_each_possible_cpu(cpu)
-		sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
-	return sum;
+	sap = rcu_dereference(sp->per_cpu_ref);
+	if (likely(sap != NULL))
+		for_each_possible_cpu(cpu)
+			sum += per_cpu_ptr(sap, cpu)->c[idx];
+	return sum + atomic_read(&sp->hardluckref[idx]);
 }
 
 /**
@@ -94,7 +111,8 @@ void cleanup_srcu_struct(struct srcu_str
 	WARN_ON(sum);  /* Leakage unless caller handles error. */
 	if (sum != 0)
 		return;
-	free_percpu(sp->per_cpu_ref);
+	if (sp->per_cpu_ref != NULL)
+		free_percpu(sp->per_cpu_ref);
 	sp->per_cpu_ref = NULL;
 }
 
@@ -105,18 +123,41 @@ void cleanup_srcu_struct(struct srcu_str
  * Counts the new reader in the appropriate per-CPU element of the
  * srcu_struct.  Must be called from process context.
  * Returns an index that must be passed to the matching srcu_read_unlock().
+ * The index is mapped to negative numbers if the srcu_struct is not and
+ * cannot be initialized.
  */
 int srcu_read_lock(struct srcu_struct *sp)
 {
 	int idx;
+	struct srcu_struct_array *sap;
 
 	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. */
+	sap = rcu_dereference(sp->per_cpu_ref);
+	if (likely(sap != NULL)) {
+		per_cpu_ptr(sap, smp_processor_id())->c[idx]++;
+		smp_mb();
+		preempt_enable();
+		return idx;
+	}
+	if (mutex_trylock(&sp->mutex)) {
+		preempt_enable();
+		if (sp->per_cpu_ref == NULL)
+			sp->per_cpu_ref = alloc_srcu_struct_percpu();
+		if (sp->per_cpu_ref == NULL) {
+			mutex_unlock(&sp->mutex);
+			preempt_disable();
+			idx = sp->completed & 0x1;
+		} else {
+			mutex_unlock(&sp->mutex);
+			return srcu_read_lock(sp);
+		}
+	}
+	atomic_inc(&sp->hardluckref[idx]);
+	smp_mb__after_atomic_inc();
 	preempt_enable();
-	return idx;
+	return -1 - idx;
 }
 
 /**
@@ -131,10 +172,16 @@ int srcu_read_lock(struct srcu_struct *s
  */
 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]--;
-	preempt_enable();
+	if (likely(idx <= 0)) {
+		preempt_disable();
+		smp_mb();
+		per_cpu_ptr(rcu_dereference(sp->per_cpu_ref),
+			    smp_processor_id())->c[idx]--;
+		preempt_enable();
+		return;
+	}
+	smp_mb__before_atomic_dec();
+	atomic_dec(&sp->hardluckref[-1 - idx]);
 }
 
 /**
@@ -158,6 +205,11 @@ void synchronize_srcu(struct srcu_struct
 	idx = sp->completed;
 	mutex_lock(&sp->mutex);
 
+	/* Initialize if not already initialized. */
+
+	if (sp->per_cpu_ref == NULL)
+		sp->per_cpu_ref = alloc_srcu_struct_percpu();
+
 	/*
 	 * Check to see if someone else did the work for us while we were
 	 * waiting to acquire the lock.  We need -two- advances of
@@ -173,65 +225,25 @@ void synchronize_srcu(struct srcu_struct
 		return;
 	}
 
-	synchronize_sched();  /* Force memory barrier on all CPUs. */
-
-	/*
-	 * 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.
-	 *
-	 * So it is now safe to do the flip.
-	 */
-
+	smp_mb();  /* ensure srcu_read_lock() sees prior change first! */
 	idx = sp->completed & 0x1;
 	sp->completed++;
 
-	synchronize_sched();  /* Force memory barrier on all CPUs. */
+	synchronize_sched();
 
 	/*
 	 * 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.
+	 * will have finished executing.  The "old" rank of counters
+	 * can therefore only decrease, never increase in value.
 	 */
 
 	while (srcu_readers_active_idx(sp, idx))
 		schedule_timeout_interruptible(1);
 
-	synchronize_sched();  /* 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.
-	 *
-	 * 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.
-	 */
+	smp_mb();  /* must see critical section prior to srcu_read_unlock() */
 
 	mutex_unlock(&sp->mutex);
 }
-
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