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Date: Tue, 10 Oct 2017 08:50:42 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: David Howells <dhowells@...hat.com>
Cc: linux-kernel@...r.kernel.org, mingo@...nel.org,
torvalds@...ux-foundation.org, mark.rutland@....com,
linux-arch@...r.kernel.org, peterz@...radead.org,
will.deacon@....com, Jonathan Corbet <corbet@....net>,
Alexander Kuleshov <kuleshovmail@...il.com>
Subject: Re: [PATCH RFC tip/core/rcu 12/15] lib/assoc_array: Remove
smp_read_barrier_depends()
On Tue, Oct 10, 2017 at 10:35:46AM +0100, David Howells wrote:
> Paul E. McKenney <paulmck@...ux.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@...ux.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@...ux.vnet.ibm.com>
Cc: Jonathan Corbet <corbet@....net>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Alexander Kuleshov <kuleshovmail@...il.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;
}
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