V4L/DVB (7714): pvrusb2: Fix hang on module removal
The pvrusb2 driver was getting had by this scenario: 1. Task A calls kthread_stop() for task B. 2. Before exiting, then Task B calls kthread_stop() for task C. The problem is, kthread_stop() wants to allocate an internal resource to itself (i.e. acquire a lock), which won't be released until kthread_stop() returns. But kthread_stop() won't return until task B is dead. But task B won't die until it finishes its call to kthread_stop() for task C, and that will block waiting on the resource already allocated inside task A. Deadlock. With the pvrusb2 driver, task A is the caller to pvr_exit(), task B is the control thread run inside of pvrusb2-context.c, and task C is any worker thread run inside of pvrusb2-hdw.c. This problem got introduced by the previous threading setup change, which was itself an attempt to fix a module tear-down race (which it actually did fix). The lesson here is that a task being waited on as part of a kthread_stop() simply cannot be allow to also issue a kthread_stop() - or we make sure not to issue the enclosing kthread_stop() until we know that the inner kthread_stop() has completed first. The solution for the pvrusb2 driver is some hackish code which changes the main control thread tear down into a two step process. This then makes it possible to delay issuing the kthread_stop() on the control thread until after we know that everything has been torn down first. (And yes, we really need that kthread_stop() because it's the only way to safely guarantee that a module-referencing kernel thread has safely returned back out of the module before we finally remove the module.) Signed-off-by: Mike Isely <isely@pobox.com> Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
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1 changed files with 18 additions and 1 deletions
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@ -35,6 +35,9 @@ static struct pvr2_context *pvr2_context_notify_first;
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static struct pvr2_context *pvr2_context_notify_last;
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static DEFINE_MUTEX(pvr2_context_mutex);
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static DECLARE_WAIT_QUEUE_HEAD(pvr2_context_sync_data);
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static DECLARE_WAIT_QUEUE_HEAD(pvr2_context_cleanup_data);
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static int pvr2_context_cleanup_flag;
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static int pvr2_context_cleaned_flag;
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static struct task_struct *pvr2_context_thread_ptr;
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@ -153,7 +156,7 @@ static void pvr2_context_check(struct pvr2_context *mp)
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static int pvr2_context_shutok(void)
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{
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return kthread_should_stop() && (pvr2_context_exist_first == NULL);
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return pvr2_context_cleanup_flag && (pvr2_context_exist_first == NULL);
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}
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@ -174,6 +177,15 @@ static int pvr2_context_thread_func(void *foo)
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pvr2_context_shutok()));
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} while (!pvr2_context_shutok());
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pvr2_context_cleaned_flag = !0;
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wake_up(&pvr2_context_cleanup_data);
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pvr2_trace(PVR2_TRACE_CTXT,"pvr2_context thread cleaned up");
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wait_event_interruptible(
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pvr2_context_sync_data,
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kthread_should_stop());
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pvr2_trace(PVR2_TRACE_CTXT,"pvr2_context thread end");
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return 0;
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@ -191,6 +203,11 @@ int pvr2_context_global_init(void)
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void pvr2_context_global_done(void)
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{
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pvr2_context_cleanup_flag = !0;
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wake_up(&pvr2_context_sync_data);
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wait_event_interruptible(
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pvr2_context_cleanup_data,
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pvr2_context_cleaned_flag);
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kthread_stop(pvr2_context_thread_ptr);
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}
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