netdev - Re: [PATCH] net: usb: Remove disruptive netif_wake_queue in rtl8150_set

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Message-ID: <CAPrAcgOb0FhWKQ6jiAVbDQZS29Thz+dXF0gdjE=7jc1df-QpvQ@mail.gmail.com>
Date: Sat, 20 Sep 2025 22:22:11 +0530
From: viswanath <viswanathiyyappan@...il.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: petkan@...leusys.com, andrew+netdev@...n.ch, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
	linux-usb@...r.kernel.org, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, skhan@...uxfoundation.org, 
	linux-kernel-mentees@...ts.linux.dev, david.hunter.linux@...il.com, 
	syzbot+78cae3f37c62ad092caa@...kaller.appspotmail.com
Subject: Re: [PATCH] net: usb: Remove disruptive netif_wake_queue in rtl8150_set_multicast

On Sat, 20 Sept 2025 at 21:00, Andrew Lunn <andrew@...n.ch> wrote:
>
> On Sat, Sep 20, 2025 at 10:20:59AM +0530, I Viswanath wrote:
> > syzbot reported WARNING in rtl8150_start_xmit/usb_submit_urb.
> > This is a possible sequence of events:
> >
> >     CPU0 (in rtl8150_start_xmit)   CPU1 (in rtl8150_start_xmit)    CPU2 (in rtl8150_set_multicast)
> >     netif_stop_queue();
> >                                                                     netif_stop_queue();
> >     usb_submit_urb();
> >                                                                     netif_wake_queue();  <-- Wakes up TX queue before it's ready
> >                                     netif_stop_queue();
> >                                     usb_submit_urb();                                    <-- Warning
> >       freeing urb
> >
> > Remove netif_wake_queue and corresponding netif_stop_queue in rtl8150_set_multicast to
> > prevent this sequence of events
>
> Please expand this sentence with an explanation of why this is
> safe. Why are these two calls not needed? The original author of this
> code thought they where needed, so you need to explain why they are
> not needed.
>
>     Andrew
>
> ---
> pw-bot: cr

Hello,

    Thanks for pointing that out. I wasn't thinking from that point of view.

    According to Documentation, rtl8150_set_multicast (the
ndo_set_rx_mode callback) should
    rely on the netif_addr_lock spinlock, not the netif_tx_lock
manipulated by netif
    stop/start/wake queue functions.

    However, There is no need to use the netif_addr_lock in the driver
directly because
    the core function (dev_set_rx_mode) invoking this function locks
and unlocks the lock
    correctly.

    Synchronization is therefore handled by the core, making it safe
to remove that lock.

    From what I have seen, every network driver assumes this for the
ndo_set_rx_mode callback.

    I am not sure what the historical context was for using the
tx_lock as the synchronization
    mechanism here but it's definitely not valid in the modern networking stack.

Thanks
Viswanath