bcache: use llist_for_each_entry_safe() in __closure_wake_up()

Commit 09b3efec ("bcache: Don't reinvent the wheel but use existing llist
API") replaces the following while loop by llist_for_each_entry(),

-
-	while (reverse) {
-		cl = container_of(reverse, struct closure, list);
-		reverse = llist_next(reverse);
-
+	llist_for_each_entry(cl, reverse, list) {
 		closure_set_waiting(cl, 0);
 		closure_sub(cl, CLOSURE_WAITING + 1);
 	}

This modification introduces a potential race by iterating a corrupted
list. Here is how it happens.

In the above modification, closure_sub() may wake up a process which is
waiting on reverse list. If this process decides to wait again by calling
closure_wait(), its cl->list will be added to another wait list. Then
when llist_for_each_entry() continues to iterate next node, it will travel
on another new wait list which is added in closure_wait(), not the
original reverse list in __closure_wake_up(). It is more probably to
happen on UP machine because the waked up process may preempt the process
which wakes up it.

Use llist_for_each_entry_safe() will fix the issue, the safe version fetch
next node before waking up a process. Then the copy of next node will make
sure list iteration stays on original reverse list.

Fixes: 09b3efec81de ("bcache: Don't reinvent the wheel but use existing llist API")
Signed-off-by: Coly Li <colyli@suse.de>
Reported-by: Michael Lyle <mlyle@lyle.org>
Reviewed-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/md/bcache/closure.c b/drivers/md/bcache/closure.c
index 7d5286b05036976059c3af6c7277a987b457c59d..1841d0359bace765cc52acd6340bb96efb5aa2ee 100644 (file)
--- a/drivers/md/bcache/closure.c
+++ b/drivers/md/bcache/closure.c
@@ -64,7 +64,7 @@ EXPORT_SYMBOL(closure_put);
 void __closure_wake_up(struct closure_waitlist *wait_list)
 {
        struct llist_node *list;
-       struct closure *cl;
+       struct closure *cl, *t;
        struct llist_node *reverse = NULL;
 
        list = llist_del_all(&wait_list->list);
@@ -73,7 +73,7 @@ void __closure_wake_up(struct closure_waitlist *wait_list)
        reverse = llist_reverse_order(list);
 
        /* Then do the wakeups */
-       llist_for_each_entry(cl, reverse, list) {
+       llist_for_each_entry_safe(cl, t, reverse, list) {
                closure_set_waiting(cl, 0);
                closure_sub(cl, CLOSURE_WAITING + 1);
        }