netdev - Re: [PATCH net] usb: cdc-wdm: fix reading stuck on device close

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Date:   Mon, 2 May 2022 12:30:39 +0200
From:   Oliver Neukum <oneukum@...e.com>
To:     Sergey Ryazanov <ryazanov.s.a@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Oliver Neukum <oneukum@...e.com>
Cc:     linux-usb@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net] usb: cdc-wdm: fix reading stuck on device close



On 01.05.22 19:58, Sergey Ryazanov wrote:
> cdc-wdm tracks whether a response reading request is in-progress and
> blocks the next request from being sent until the previous request is
> completed. As soon as last user closes the cdc-wdm device file, the
> driver cancels any ongoing requests, resets the pending response
> counter, but leaves the response reading in-progress flag
> (WDM_RESPONDING) untouched.
>
> So if the user closes the device file during the response receive
> request is being performed, no more data will be obtained from the
> modem. The request will be cancelled, effectively preventing the
> WDM_RESPONDING flag from being reseted. Keeping the flag set will
> prevent a new response receive request from being sent, permanently
> blocking the read path. The read path will staying blocked until the
> module will be reloaded or till the modem will be re-attached.
>
> This stuck has been observed with a Huawei E3372 modem attached to an
> OpenWrt router and using the comgt utility to set up a network
> connection.
>
> Fix this issue by clearing the WDM_RESPONDING flag on the device file
> close.
>
> Without this fix, the device reading stuck can be easily reproduced in a
> few connection establishing attempts. With this fix, a load test for
> modem connection re-establishing worked for several hours without any
> issues.
>
> Fixes: 922a5eadd5a3 ("usb: cdc-wdm: Fix race between autosuspend and
> reading from the device")
> Signed-off-by: Sergey Ryazanov <ryazanov.s.a@...il.com>
>
Acked-by: Oliver Neukum <oneukum@...e.com>