[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 246feae16c602ff3f4a1ad1024fd64334e1cbc39..2540dfaa3e385def27a44284bf3e552f986ee15d 100644 (file)
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1487,20 +1487,22 @@ cfq_cic_rb_lookup(struct cfq_data *cfqd, struct io_context *ioc)
 {
        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 @@ cfq_cic_link(struct cfq_data *cfqd, struct io_context *ioc,
        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 @@ restart:
        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();