closures: fix a race on wakeup from closure_sync

The race was when a thread using closure_sync() notices cl->s->done == 1
before the thread calling closure_put() calls wake_up_process(). Then,
it's possible for that thread to return and exit just before
wake_up_process() is called - so we're trying to wake up a process that
no longer exists.

rcu_read_lock() is sufficient to protect against this, as there's an rcu
barrier somewhere in the process teardown path.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Acked-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 8 insertions, 2 deletions

diff --git a/drivers/md/bcache/closure.c b/drivers/md/bcache/closure.c
index 73f5319295bc..c12cd809ab19 100644
--- a/drivers/md/bcache/closure.c
+++ b/drivers/md/bcache/closure.c
@@ -105,8 +105,14 @@ struct closure_syncer {
 
 static void closure_sync_fn(struct closure *cl)
 {
-        cl->s->done = 1;
-        wake_up_process(cl->s->task);
+        struct closure_syncer *s = cl->s;
+        struct task_struct *p;
+
+        rcu_read_lock();
+        p = READ_ONCE(s->task);
+        s->done = 1;
+        wake_up_process(p);
+        rcu_read_unlock();
 }
 
 void __sched __closure_sync(struct closure *cl)