Re: [PATCH v3] usb: gadget: u_ether: Use __netif_rx() in rx_callback()

["Hubert Wiśniewski" <hubert.wisniewski.25632@gmail.com>]

On Thu, 2024-09-26 at 21:39 +0200, Hubert Wiśniewski wrote:
> I'm a bit at loss here. The deadlock seems to be unrelated to netif_rx()
> (which is not being called in the interrupt context after all), yet
> replacing it with __netif_rx() fixes the lockup (though a warning is still
> generated, which suggests that the patch does not completely fix the
> issue).

Well, never mind. After some investigation, I think the problem is as
follows:

1. musb_g_giveback() releases the musb lock using spin_unlock(). The lock
is now released, but hardirqs are still disabled.

2. Then, usb_gadget_giveback_request() is called, which in turn calls
rx_complete(). This does not happen in the interrupt context, so netif_rx()
disables bottom havles, then enables them using local_bh_enable().

3. This leads to calling __local_bh_enable_ip(), which gives off a warning
(the first backtrace) that hardirqs are disabled. Then, hardirqs are
disabled (again?), and then enabled (as they should have been in the first
place).

4. After usb_gadget_giveback_request() returns, musb_g_giveback() acquires
the musb lock using spin_lock(). This does not disable hardirqs, so they
are still enabled.

5. While the musb lock is acquired, an interrupt occurs. It is handled by
dsps_interrupt(), which acquires the musb lock. A deadlock occurs.

Replacing netif_rx() with __netif_rx() apparently fixes this part, as it
does not lead to any change of hardirq state. There is still one problem
though: rx_complete() is usually called from the interrupt context, except
when the network interface is brought up.

I think one solution would be to make musb_g_giveback() use
spin_unlock_irqrestore() and spin_lock_irqsave(), but I would need to pass
the flags to it somehow. Also, I am not sure how that would influence other
drivers using musb.

-- 
Hubert Wiśniewski <hubert.wisniewski.25632@gmail.com>