[PATCH AUTOSEL 5.2 144/185] closures: fix a race on wakeup from closure_sync

Sasha Levin
2019-09-22T19:23:41Z

From: Kent Overstreet <kent.overstreet@gmail.com>

[ Upstream commit a22a9602b88fabf10847f238ff81fde5f906fef7 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/md/bcache/closure.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/md/bcache/closure.c b/drivers/md/bcache/closure.c
index 73f5319295bc9..c12cd809ab193 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)
--
2.20.1