Re: [PATCH] Bluetooth: Fix hidp disconnect deadlock

* Ilia, Kolominsky [off-list ref] [2011-06-30 16:55:52 +0200]:

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-----Original Message-----
From: Peter Hurley [mailto:peter@hurleysoftware.com]
Sent: Thursday, June 30, 2011 5:35 PM
To: Gustavo F. Padovan
Cc: Ilia, Kolominsky; linux-bluetooth
Subject: Re: [PATCH] Bluetooth: Fix hidp disconnect deadlock

On Wed, 2011-06-29 at 16:52 -0400, Gustavo F. Padovan wrote:

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* Gustavo F. Padovan [off-list ref] [2011-06-29 17:24:56 -

0300]:

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* Ilia, Kolominsky [off-list ref] [2011-06-26 09:16:58 +0200]:

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Hi!
IMHO the fix isnt good due to possible race condition which
will destroy session/task objects - either by a call to

kthread_stop

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from the timer func or reentry to hidp_del_connection() on
smp platforms.

....

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This should fix the timer issue. Please test.

	Gustavo

Hi Ilia & Gustavo,

After Ilia pointed out the problem with the timer function, I went back
and reviewed *all* the synchronization code relevant to the hid session
thread.

A number of problems were introduced with commit aabf6f89 - when the
session thread was converted from a kernel_thread to a kthread.
Although
a kthread is a better choice for representing the session thread, the
naive conversion of atomic/wakeup to kthread_stop() was inappropriate.

kthread_stop() has usage semantics that different significantly from
atomic/wakeup. As we already know, because kthread_stop() blocks on
thread completion, it can introduce deadlocks in code that already uses
exclusion mechanisms. Even with Ilia's new patch, consider the
following
sequence:

Thread 0                 Thread 1                 Thread 2
in hidp_del_connection                            in hidp session
  claim r/w sem                                     .
                         timer triggers             .
                           kthread_stop() --------->.
                           *blocks on thread 2*     exits loop
                                                    *blocks for r/w

Guys, as far as I know - it is not possible to use kthread_stop from
The timer func - since it may sleep and we are in soft_irq...
What I did is added kthread_stop_async in the kthread use it in both 
Timer func and hidp_del_connection(). I am still polishing the code
though...

Yes, you right, we really can't call kthread_stop there.

Gustavo, is adding the new api to kthread feasible?
How to upstream such change?

I don't know the kthread internals to tell you if this is feasible or not. You
have to send the patch to the maintainer of kthread
(./scripts/get_maintainer.pl tells you the maintainer) and explain why you
need this to him.

But we also need a solution in the short term, this week, in order to fix this
in the up coming 3.0 release.

	Gustavo