From a5f3d8a5eaaf917878f07998e6f1ea46024e6bab Mon Sep 17 00:00:00 2001 From: Coly Li Date: Tue, 26 Sep 2017 17:54:12 +0800 Subject: [PATCH] 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 Reported-by: Michael Lyle Reviewed-by: Byungchul Park Signed-off-by: Jens Axboe --- 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); } -- 2.20.1