linux-kernel - Re: [PATCH 4/6] usbip: fix stub_dev usbip_sockfd

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Message-ID: <47dbbf10-368c-6e45-5eac-c57b75f7ae9c@i-love.sakura.ne.jp>
Date:   Wed, 10 Mar 2021 10:02:42 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Shuah Khan <skhan@...uxfoundation.org>, shuah@...nel.org,
        valentina.manea.m@...il.com, gregkh@...uxfoundation.org
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] usbip: fix stub_dev usbip_sockfd_store() races
 leading to gpf

On 2021/03/10 9:29, Shuah Khan wrote:
>> It is not a large grain lock. Since event_handler() is exclusively executed, this lock
>> does _NOT_ block event_handler() unless attach/detach operations run concurrently.
>>
>>>
> 
> event handler queues the events. It shouldn't be blocked by attach
> and detach. The events could originate for various reasons during
> the host and vhci operations. I don't like using this lock for
> attach and detach.

How can attach/detach deadlock event_handler()?
event_handler() calls e.g. vhci_shutdown_connection() via ud->eh_ops.shutdown(ud).
vhci_shutdown_connection() e.g. waits for termination of tx/rx threads via kthread_stop_put().
event_handler() is already blocked by detach operation.
How it can make situation worse to wait for creation of tx/rx threads in attach operation?