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.
Signed-off-by: Coly Li <colyli@xxxxxxx>
Reported-by: Michael Lyle <mlyle@xxxxxxxx>
---
drivers/md/bcache/closure.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/md/bcache/closure.c b/drivers/md/bcache/closure.c
index 7d5286b05036..1841d0359bac 100644
--- 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);
}
--
2.13.5
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