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Subject[PATCH 2/2] dvb_frontend: eliminate blocking wait in dvb_unregister_frontend()
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The wait_event() call in dvb_unregister_frontend() waits synchronously
for other tasks to free a file descriptor, but it does that while
holding several mutexes. That alone is a bad idea, but if one user
process happens to keep a (defunct) file descriptor open indefinitely,
the kernel will correctly detect a hung task:

INFO: task kworker/0:1:314 blocked for more than 30 seconds.
Not tainted 4.7.0-rc1-hosting+ #50
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
kworker/0:1 D ffff88003daf7a50 0 314 2 0x00000000
Workqueue: usb_hub_wq hub_event
ffff88003daf7a50 0000000000000296 ffff88003daf7a30 ffff88003fc13f98
ffff88003dadce00 ffff88003daf8000 ffff88003e3fc010 ffff88003d48d4f8
ffff88003e3b5030 ffff88003e3f8898 ffff88003daf7a68 ffffffff810cf860
Call Trace:
[<ffffffff810cf860>] schedule+0x30/0x80
[<ffffffff812f88d3>] dvb_unregister_frontend+0x93/0xc0
[<ffffffff8107a000>] ? __wake_up_common+0x80/0x80
[<ffffffff813019c7>] dvb_usb_adapter_frontend_exit+0x37/0x70
[<ffffffff81300614>] dvb_usb_exit+0x34/0xb0
[<ffffffff81300d4a>] dvb_usb_device_exit+0x3a/0x50
[<ffffffff81302dc2>] pctv452e_usb_disconnect+0x52/0x60
[<ffffffff81295a07>] usb_unbind_interface+0x67/0x1e0
[<ffffffff810609f3>] ? __blocking_notifier_call_chain+0x53/0x70
[<ffffffff8127ba67>] __device_release_driver+0x77/0x110
[<ffffffff8127c2d3>] device_release_driver+0x23/0x30
[<ffffffff8127ab5d>] bus_remove_device+0x10d/0x150
[<ffffffff8127879b>] device_del+0x13b/0x260
[<ffffffff81299dea>] ? usb_remove_ep_devs+0x1a/0x30
[<ffffffff8129468e>] usb_disable_device+0x9e/0x1e0
[<ffffffff8128bb09>] usb_disconnect+0x89/0x260
[<ffffffff8128db8d>] hub_event+0x30d/0xfc0
[<ffffffff81059475>] process_one_work+0x1c5/0x4a0
[<ffffffff8105940c>] ? process_one_work+0x15c/0x4a0
[<ffffffff81059799>] worker_thread+0x49/0x480
[<ffffffff81059750>] ? process_one_work+0x4a0/0x4a0
[<ffffffff81059750>] ? process_one_work+0x4a0/0x4a0
[<ffffffff8105f65e>] kthread+0xee/0x110
[<ffffffff810400bf>] ret_from_fork+0x1f/0x40
[<ffffffff8105f570>] ? __kthread_unpark+0x70/0x70
5 locks held by kworker/0:1/314:
#0: ("usb_hub_wq"){......}, at: [<ffffffff8105940c>] process_one_work+0x15c/0x4a0
#1: ((&hub->events)){......}, at: [<ffffffff8105940c>] process_one_work+0x15c/0x4a0
#2: (&dev->mutex){......}, at: [<ffffffff8128d8cb>] hub_event+0x4b/0xfc0
#3: (&dev->mutex){......}, at: [<ffffffff8128bad2>] usb_disconnect+0x52/0x260
#4: (&dev->mutex){......}, at: [<ffffffff8127c2cb>] device_release_driver+0x1b/0x30

This patch removes the blocking wait, and postpones the kfree() call
until all file handles have been closed by using struct kref.

Signed-off-by: Max Kellermann <max@duempel.org>
---
drivers/media/dvb-core/dvb_frontend.c | 33 +++++++++++++++++++++++++--------
1 file changed, 25 insertions(+), 8 deletions(-)

diff --git a/drivers/media/dvb-core/dvb_frontend.c b/drivers/media/dvb-core/dvb_frontend.c
index c014261..be99c8d 100644
--- a/drivers/media/dvb-core/dvb_frontend.c
+++ b/drivers/media/dvb-core/dvb_frontend.c
@@ -99,6 +99,7 @@ MODULE_PARM_DESC(dvb_mfe_wait_time, "Wait up to <mfe_wait_time> seconds on open(
static DEFINE_MUTEX(frontend_mutex);

struct dvb_frontend_private {
+ struct kref refcount;

/* thread/frontend values */
struct dvb_device *dvbdev;
@@ -137,6 +138,23 @@ struct dvb_frontend_private {
#endif
};

+static void dvb_frontend_private_free(struct kref *ref)
+{
+ struct dvb_frontend_private *fepriv =
+ container_of(ref, struct dvb_frontend_private, refcount);
+ kfree(fepriv);
+}
+
+static void dvb_frontend_private_put(struct dvb_frontend_private *fepriv)
+{
+ kref_put(&fepriv->refcount, dvb_frontend_private_free);
+}
+
+static void dvb_frontend_private_get(struct dvb_frontend_private *fepriv)
+{
+ kref_get(&fepriv->refcount);
+}
+
static void dvb_frontend_wakeup(struct dvb_frontend *fe);
static int dtv_get_frontend(struct dvb_frontend *fe,
struct dtv_frontend_properties *c,
@@ -2543,6 +2561,8 @@ static int dvb_frontend_open(struct inode *inode, struct file *file)
fepriv->events.eventr = fepriv->events.eventw = 0;
}

+ dvb_frontend_private_get(fepriv);
+
if (adapter->mfe_shared)
mutex_unlock (&adapter->mfe_lock);
return ret;
@@ -2591,6 +2611,8 @@ static int dvb_frontend_release(struct inode *inode, struct file *file)
fe->ops.ts_bus_ctrl(fe, 0);
}

+ dvb_frontend_private_put(fepriv);
+
return ret;
}

@@ -2679,6 +2701,8 @@ int dvb_register_frontend(struct dvb_adapter* dvb,
}
fepriv = fe->frontend_priv;

+ kref_init(&fepriv->refcount);
+
sema_init(&fepriv->sem, 1);
init_waitqueue_head (&fepriv->wait_queue);
init_waitqueue_head (&fepriv->events.wait_queue);
@@ -2713,18 +2737,11 @@ int dvb_unregister_frontend(struct dvb_frontend* fe)

mutex_lock(&frontend_mutex);
dvb_frontend_stop (fe);
- mutex_unlock(&frontend_mutex);
-
- if (fepriv->dvbdev->users < -1)
- wait_event(fepriv->dvbdev->wait_queue,
- fepriv->dvbdev->users==-1);
-
- mutex_lock(&frontend_mutex);
dvb_unregister_device (fepriv->dvbdev);

/* fe is invalid now */
- kfree(fepriv);
mutex_unlock(&frontend_mutex);
+ dvb_frontend_private_put(fepriv);
return 0;
}
EXPORT_SYMBOL(dvb_unregister_frontend);
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