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

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Date:   Sun,  1 May 2022 20:58:28 +0300
From:   Sergey Ryazanov <ryazanov.s.a@...il.com>
To:     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: [PATCH net] usb: cdc-wdm: fix reading stuck on device close

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>
---

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(+)

diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c
index 7f2c83f299d3..eebe782380fb 100644
--- a/drivers/usb/class/cdc-wdm.c
+++ b/drivers/usb/class/cdc-wdm.c
@@ -774,6 +774,7 @@ static int wdm_release(struct inode *inode, struct file *file)
 			poison_urbs(desc);
 			spin_lock_irq(&desc->iuspin);
 			desc->resp_count = 0;
+			clear_bit(WDM_RESPONDING, &desc->flags);
 			spin_unlock_irq(&desc->iuspin);
 			desc->manage_power(desc->intf, 0);
 			unpoison_urbs(desc);
-- 
2.35.1