Re: [RFC][PATCH] netconsole: avoid deadlock on printk from driver code

From: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Date: Wed, 13 Aug 2008 13:59:43 +0400

On Wed, Aug 13, 2008 at 11:53:24AM +0200, Vegard Nossum wrote:
I encountered a hard-to-debug deadlock when I pulled out the plug of my
RealTek 8139 which was also running netconsole: The driver wants to print
a "link down" message. However, this triggers netconsole, which wants to
print the message using the same device. Here is a backtrace:

[<c05916b6>] _spin_lock_irqsave+0x76/0x90
[<c035b255>] rtl8139_start_xmit+0x65/0x130 <-- spin_lock(&tp->lock)
[<c04c5e28>] netpoll_send_skb+0x158/0x1a0
[<c04c62fb>] netpoll_send_udp+0x1db/0x1f0
[<c037c70c>] write_msg+0x8c/0xc0
[<c0135883>] __call_console_drivers+0x53/0x60
[<c01358db>] _call_console_drivers+0x4b/0x90
[<c0135a25>] release_console_sem+0xc5/0x1f0
[<c0135f0b>] vprintk+0x1ab/0x3e0
[<c013615b>] printk+0x1b/0x20
[<c0349736>] mii_check_media+0x196/0x1e0
[<c03597f4>] rtl_check_media+0x24/0x30
[<c035a0ea>] rtl8139_interrupt+0x42a/0x4a0 <-- spin_lock(&tp->lock)
[<c01716d8>] handle_IRQ_event+0x28/0x70
[<c0172d9b>] handle_fasteoi_irq+0x6b/0xe0
[<c0107128>] do_IRQ+0x48/0xa0

The least invasive fix is to detect that we're trying to re-enter the
driver code. We provide a netdev_busy() function which can be used to
determine whether a deadlock can occur if we try to transmit another
packet.

Note that this may lead to lost messages if the driver is active on
another CPU while we try to use the same device for netconsole.

This sucks.

It's also the wrong fix.

As a quicker and more palatable solution, print your link status
message in some kind of deferred context where you can have the
lock not held or similar.
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