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

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Date:   Sun, 1 May 2022 20:09:14 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sergey Ryazanov <ryazanov.s.a@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Oliver Neukum <oneukum@...e.com>, linux-usb@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH net] usb: cdc-wdm: fix reading stuck on device close

On Sun, May 01, 2022 at 08:58:28PM +0300, 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")

Nit, Fixes: lines should only be one line, I'll fix this up when
applying it.

> Signed-off-by: Sergey Ryazanov <ryazanov.s.a@...il.com>
> ---
> 
> Technically, cdc-wdm belongs to the USB subsystem even though it serves
> WWAN devices. I think this fix should be applied (backported) to LTS
> versions too. So I targeted it to the 'net' tree, but send it to both
> lists to get a feedback. Greg, Jakub, what is the best tree for this
> fix?
> 
> ---
>  drivers/usb/class/cdc-wdm.c | 1 +
>  1 file changed, 1 insertion(+)

scripts/get_maintainer.pl is pretty clear this goes through the USB
tree.  I'll queue it up in a few days, thanks,

greg k-h