[PATCH] w1: fix deadloop in __w1_remove_master_device()

[Yang Yingliang]

I got a deadloop report while doing device(ds2482) add/remove test:

  [  162.241881] w1_master_driver w1_bus_master1: Waiting for w1_bus_master1 to become free: refcnt=1.
  [  163.272251] w1_master_driver w1_bus_master1: Waiting for w1_bus_master1 to become free: refcnt=1.
  [  164.296157] w1_master_driver w1_bus_master1: Waiting for w1_bus_master1 to become free: refcnt=1.
  ...

__w1_remove_master_device() can't return, because the dev->refcnt is not zero.

w1_add_master_device()			|
  w1_alloc_dev()			|
    atomic_set(&dev->refcnt, 2)		|
  kthread_run()				|
					|__w1_remove_master_device()
					|  kthread_stop()
  // KTHREAD_SHOULD_STOP is set,	|
  // threadfn(w1_process) won't be	|
  // called.				|
  kthread()				|
					|  // refcnt will never be 0, it's deadloop.
					|  while (atomic_read(&dev->refcnt)) {...}

After calling w1_add_master_device(), w1_process() is not really
invoked, before w1_process() starting, if kthread_stop() is called
in __w1_remove_master_device(), w1_process() will never be called,
the refcnt can not be decreased, then it causes deadloop in remove
function because of non-zero refcnt.

We need to make sure w1_process() is really started, so move the
set refcnt into w1_process() to fix this problem.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Yang Yingliang <yangyingliang@xxxxxxxxxx>
---
 drivers/w1/w1.c     | 2 ++
 drivers/w1/w1_int.c | 5 ++---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/w1/w1.c b/drivers/w1/w1.c
index f2ae2e563dc5..8b35fae4cd61 100644
--- a/drivers/w1/w1.c
+++ b/drivers/w1/w1.c
@@ -1166,6 +1166,8 @@ int w1_process(void *data)
 	/* remainder if it woke up early */
 	unsigned long jremain = 0;
 
+	atomic_inc(&dev->refcnt);
+
 	for (;;) {
 
 		if (!jremain && dev->search_count) {
diff --git a/drivers/w1/w1_int.c b/drivers/w1/w1_int.c
index b3e1792d9c49..3a71c5eb2f83 100644
--- a/drivers/w1/w1_int.c
+++ b/drivers/w1/w1_int.c
@@ -51,10 +51,9 @@ static struct w1_master *w1_alloc_dev(u32 id, int slave_count, int slave_ttl,
 	dev->search_count	= w1_search_count;
 	dev->enable_pullup	= w1_enable_pullup;
 
-	/* 1 for w1_process to decrement
-	 * 1 for __w1_remove_master_device to decrement
+	/* For __w1_remove_master_device to decrement
 	 */
-	atomic_set(&dev->refcnt, 2);
+	atomic_set(&dev->refcnt, 1);
 
 	INIT_LIST_HEAD(&dev->slist);
 	INIT_LIST_HEAD(&dev->async_list);
-- 
2.25.1