Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove smp_read_barrier_depends()

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

On Tue, Oct 10, 2017 at 10:35:46AM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> >  static inline void *assoc_array_ptr_to_leaf(const struct assoc_array_ptr *x)
> >  {
> > -	return (void *)((unsigned long)x & ~ASSOC_ARRAY_PTR_TYPE_MASK);
> > +	return (void *)((unsigned long)READ_ONCE(x) & /* Address dependency. */
> > +		~ASSOC_ARRAY_PTR_TYPE_MASK);
> >  }
> 
> This is the wrong place to do this.  assoc_array_ptr_to_leaf() is effectively
> no more than a special cast; it removes a metadata bit from a pointer.  x is
> the value we're modifying, not *x, and x was read by the caller.

OK, so in at least one of these cases, the barrier is already provided
by the ACCESS_ONCE(array->root) in assoc_array_walk().  I took a quick
look at the other instances of ->root and convinced myself that they
were update-side accesses, but please let me know if I am missing
something.

I marked the ACCESS_ONCE() instances that seemed to me to be heading
address-dependency chains, please let me know how I did.  ;-)

> >  static inline
> >  unsigned long __assoc_array_ptr_to_meta(const struct assoc_array_ptr *x)
> >  {
> > -	return (unsigned long)x &
> > +	return (unsigned long)READ_ONCE(x) & /* Address dependency. */
> >  		~(ASSOC_ARRAY_PTR_SUBTYPE_MASK | ASSOC_ARRAY_PTR_TYPE_MASK);
> >  }
> 
> Ditto.
> 
> > -		ptr = READ_ONCE(node->slots[slot]);
> > +		ptr = READ_ONCE(node->slots[slot]); /* Address dependency. */
> >  		has_meta |= (unsigned long)ptr;
> >  		if (ptr && assoc_array_ptr_is_leaf(ptr)) {
> > -			/* We need a barrier between the read of the pointer
> > -			 * and dereferencing the pointer - but only if we are
> > -			 * actually going to dereference it.
> > -			 */
> > -			smp_read_barrier_depends();
> > -
> 
> For example, you can see the READ_ONCE() here; that is already done.
> 
> Can you also not just delete the comment, but rephrase it to indicate that a
> barrier is necessary and it's done by READ_ONCE(), please?

So, is the patch below at least somewhat less confused?  ;-)

							Thanx, Paul

------------------------------------------------------------------------

commit a25170d6998c587b296c75be09b49b5e31bc515b
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Mon Oct 9 11:39:57 2017 -0700

    lib/assoc_array: Remove smp_read_barrier_depends()
    
    Now that smp_read_barrier_depends() is implied by READ_ONCE(), adding
    READ_ONCE() to assoc_array_ptr_to_leaf() and __assoc_array_ptr_to_meta()
    allows the several smp_read_barrier_depends() calls to be removed from
    lib/assoc_array.c.  This commit makes this change.
    
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Cc: Mark Rutland <mark.rutland@arm.com>
    Cc: Alexander Kuleshov <kuleshovmail@gmail.com>

diff --git a/lib/assoc_array.c b/lib/assoc_array.c
index fe7953aead82..7e483294f7d7 100644
--- a/lib/assoc_array.c
+++ b/lib/assoc_array.c
@@ -38,12 +38,10 @@ static int assoc_array_subtree_iterate(const struct assoc_array_ptr *root,
 	if (assoc_array_ptr_is_shortcut(cursor)) {
 		/* Descend through a shortcut */
 		shortcut = assoc_array_ptr_to_shortcut(cursor);
-		smp_read_barrier_depends();
-		cursor = READ_ONCE(shortcut->next_node);
+		cursor = READ_ONCE(shortcut->next_node); /* Address dependency. */
 	}
 
 	node = assoc_array_ptr_to_node(cursor);
-	smp_read_barrier_depends();
 	slot = 0;
 
 	/* We perform two passes of each node.
@@ -55,15 +53,12 @@ static int assoc_array_subtree_iterate(const struct assoc_array_ptr *root,
 	 */
 	has_meta = 0;
 	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
-		ptr = READ_ONCE(node->slots[slot]);
+		ptr = READ_ONCE(node->slots[slot]); /* Address dependency. */
 		has_meta |= (unsigned long)ptr;
 		if (ptr && assoc_array_ptr_is_leaf(ptr)) {
-			/* We need a barrier between the read of the pointer
-			 * and dereferencing the pointer - but only if we are
-			 * actually going to dereference it.
+			/* We need a barrier between the read of the pointer,
+			 * which is supplied by the above READ_ONCE().
 			 */
-			smp_read_barrier_depends();
-
 			/* Invoke the callback */
 			ret = iterator(assoc_array_ptr_to_leaf(ptr),
 				       iterator_data);
@@ -86,10 +81,8 @@ static int assoc_array_subtree_iterate(const struct assoc_array_ptr *root,
 
 continue_node:
 	node = assoc_array_ptr_to_node(cursor);
-	smp_read_barrier_depends();
-
 	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
-		ptr = READ_ONCE(node->slots[slot]);
+		ptr = READ_ONCE(node->slots[slot]); /* Address dependency. */
 		if (assoc_array_ptr_is_meta(ptr)) {
 			cursor = ptr;
 			goto begin_node;
@@ -98,16 +91,15 @@ static int assoc_array_subtree_iterate(const struct assoc_array_ptr *root,
 
 finished_node:
 	/* Move up to the parent (may need to skip back over a shortcut) */
-	parent = READ_ONCE(node->back_pointer);
+	parent = READ_ONCE(node->back_pointer); /* Address dependency. */
 	slot = node->parent_slot;
 	if (parent == stop)
 		return 0;
 
 	if (assoc_array_ptr_is_shortcut(parent)) {
 		shortcut = assoc_array_ptr_to_shortcut(parent);
-		smp_read_barrier_depends();
 		cursor = parent;
-		parent = READ_ONCE(shortcut->back_pointer);
+		parent = READ_ONCE(shortcut->back_pointer); /* Address dependency. */
 		slot = shortcut->parent_slot;
 		if (parent == stop)
 			return 0;
@@ -147,7 +139,7 @@ int assoc_array_iterate(const struct assoc_array *array,
 					void *iterator_data),
 			void *iterator_data)
 {
-	struct assoc_array_ptr *root = READ_ONCE(array->root);
+	struct assoc_array_ptr *root = READ_ONCE(array->root); /* Address dependency. */
 
 	if (!root)
 		return 0;
@@ -194,7 +186,7 @@ assoc_array_walk(const struct assoc_array *array,
 
 	pr_devel("-->%s()\n", __func__);
 
-	cursor = READ_ONCE(array->root);
+	cursor = READ_ONCE(array->root);  /* Address dependency. */
 	if (!cursor)
 		return assoc_array_walk_tree_empty;
 
@@ -216,11 +208,9 @@ assoc_array_walk(const struct assoc_array *array,
 
 consider_node:
 	node = assoc_array_ptr_to_node(cursor);
-	smp_read_barrier_depends();
-
 	slot = segments >> (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
 	slot &= ASSOC_ARRAY_FAN_MASK;
-	ptr = READ_ONCE(node->slots[slot]);
+	ptr = READ_ONCE(node->slots[slot]); /* Address dependency. */
 
 	pr_devel("consider slot %x [ix=%d type=%lu]\n",
 		 slot, level, (unsigned long)ptr & 3);
@@ -254,7 +244,6 @@ assoc_array_walk(const struct assoc_array *array,
 	cursor = ptr;
 follow_shortcut:
 	shortcut = assoc_array_ptr_to_shortcut(cursor);
-	smp_read_barrier_depends();
 	pr_devel("shortcut to %d\n", shortcut->skip_to_level);
 	sc_level = level + ASSOC_ARRAY_LEVEL_STEP;
 	BUG_ON(sc_level > shortcut->skip_to_level);
@@ -294,7 +283,7 @@ assoc_array_walk(const struct assoc_array *array,
 	} while (sc_level < shortcut->skip_to_level);
 
 	/* The shortcut matches the leaf's index to this point. */
-	cursor = READ_ONCE(shortcut->next_node);
+	cursor = READ_ONCE(shortcut->next_node); /* Address dependency. */
 	if (((level ^ sc_level) & ~ASSOC_ARRAY_KEY_CHUNK_MASK) != 0) {
 		level = sc_level;
 		goto jumped;
@@ -330,21 +319,19 @@ void *assoc_array_find(const struct assoc_array *array,
 	    assoc_array_walk_found_terminal_node)
 		return NULL;
 
-	node = result.terminal_node.node;
-	smp_read_barrier_depends();
+	node = READ_ONCE(result.terminal_node.node); /* Address dependency. */
 
 	/* If the target key is available to us, it's has to be pointed to by
 	 * the terminal node.
 	 */
 	for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
-		ptr = READ_ONCE(node->slots[slot]);
+		ptr = READ_ONCE(node->slots[slot]); /* Address dependency. */
 		if (ptr && assoc_array_ptr_is_leaf(ptr)) {
 			/* We need a barrier between the read of the pointer
 			 * and dereferencing the pointer - but only if we are
 			 * actually going to dereference it.
 			 */
 			leaf = assoc_array_ptr_to_leaf(ptr);
-			smp_read_barrier_depends();
 			if (ops->compare_object(leaf, index_key))
 				return (void *)leaf;
 		}