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From:   Prashanth K <quic_prashk@quicinc.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CC:     Xiu Jianfeng <xiujianfeng@huawei.com>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        <linux-usb@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Prashanth K <quic_prashk@quicinc.com>
Subject: [PATCH] usb: gadget: u_serial: Avoid spinlock recursion in __gs_console_push
Date:   Tue, 9 May 2023 18:57:52 +0530
Message-ID: <1683638872-6885-1-git-send-email-quic_prashk@quicinc.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

When serial console over USB is enabled, gs_console_connect
queues gs_console_work, where it acquires the spinlock and
queues the usb request, and this request goes to gadget layer.
Now consider a situation where gadget layer prints something
to dmesg, this will eventually call gs_console_write() which
requires cons->lock. And this causes spinlock recursion. Avoid
this by excluding usb_ep_queue from the spinlock.

 spin_lock_irqsave //needs cons->lock
 gs_console_write
	.
	.
 _printk
 __warn_printk
 dev_warn/pr_err
	.
	.
 [USB Gadget Layer]
	.
	.
 usb_ep_queue
 gs_console_work
 __gs_console_push // acquires cons->lock
 process_one_work

Signed-off-by: Prashanth K <quic_prashk@quicinc.com>
---
 drivers/usb/gadget/function/u_serial.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index a0ca47f..1bc96ff 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -916,8 +916,11 @@ static void __gs_console_push(struct gs_console *cons)
 	}
 
 	req->length = size;
+
+	spin_unlock_irq(&cons->lock);
 	if (usb_ep_queue(ep, req, GFP_ATOMIC))
 		req->length = 0;
+	spin_lock_irq(&cons->lock);
 }
 
 static void gs_console_work(struct work_struct *work)
-- 
2.7.4