[PATCH] cfq: Further rbtree traversal and cfq_exit_queue() race fix
In current code, we are re-reading cic->key after dead cic->key check.
So, in theory, it may really re-read *after* cfq_exit_queue() seted NULL.
To avoid race, we copy it to stack, then use it. With this change, I
guess gcc will assign cic->key to a register or stack, and it wouldn't
be re-readed.
Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Jens Axboe <axboe@suse.de>
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 246feae..2540dfa 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1487,20 +1487,22 @@
{
struct rb_node *n;
struct cfq_io_context *cic;
- void *key = cfqd;
+ void *k, *key = cfqd;
restart:
n = ioc->cic_root.rb_node;
while (n) {
cic = rb_entry(n, struct cfq_io_context, rb_node);
- if (unlikely(!cic->key)) {
+ /* ->key must be copied to avoid race with cfq_exit_queue() */
+ k = cic->key;
+ if (unlikely(!k)) {
cfq_drop_dead_cic(ioc, cic);
goto restart;
}
- if (key < cic->key)
+ if (key < k)
n = n->rb_left;
- else if (key > cic->key)
+ else if (key > k)
n = n->rb_right;
else
return cic;
@@ -1516,6 +1518,7 @@
struct rb_node **p;
struct rb_node *parent;
struct cfq_io_context *__cic;
+ void *k;
cic->ioc = ioc;
cic->key = cfqd;
@@ -1527,14 +1530,16 @@
while (*p) {
parent = *p;
__cic = rb_entry(parent, struct cfq_io_context, rb_node);
- if (unlikely(!__cic->key)) {
+ /* ->key must be copied to avoid race with cfq_exit_queue() */
+ k = __cic->key;
+ if (unlikely(!k)) {
cfq_drop_dead_cic(ioc, cic);
goto restart;
}
- if (cic->key < __cic->key)
+ if (cic->key < k)
p = &(*p)->rb_left;
- else if (cic->key > __cic->key)
+ else if (cic->key > k)
p = &(*p)->rb_right;
else
BUG();