netdev - Re: netpoll: raspberrypi [4 5] driver locking woes

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Message-ID: <cbc6389e-069e-4f59-8544-fa59678e401b@linux.dev>
Date: Thu, 28 Aug 2025 10:57:42 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Mike Galbraith <efault@....de>, Robert Hancock <robert.hancock@...ian.com>
Cc: Breno Leitao <leitao@...ian.org>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: netpoll: raspberrypi [4 5] driver locking woes

Hi Mike,

On 8/27/25 12:02, Mike Galbraith wrote:
> Unexpected addendum to done deal datapoint, so off list.
> 
> On Tue, 2025-08-26 at 11:49 +0200, Mike Galbraith wrote:
>> 
>> The pi5 gripe fix is equally trivial, but submitting that is pointless
>> given there's something else amiss in fingered commit.  This is all of
>> the crash info that escapes the box w/wo gripes silenced.
>> 
>> [   51.688868] sysrq: Trigger a crash
>> [   51.688892] Kernel panic - not syncing: sysrq triggered crash
>> [   51.698066] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.17.0-rc2-v8-lockdep #533 PREEMPTLAZY
>> [   51.707234] Hardware name: Raspberry Pi 5 Model B Rev 1.0 (DT)
>> [   51.713085] Call trace:
>> [   51.715532]  show_stack+0x20/0x38 (C)
>> [   51.719206]  dump_stack_lvl+0x38/0xd0
>> [   51.722878]  dump_stack+0x18/0x28
>> 
>> That aspect is a punt and run atm (time.. and dash of laziness:).
> 
> Plan was to end datapoint thread, but after booting pi5's 6.12 kernel,
> for some reason I fired up netconsole.. and box promptly exhibited the
> netpoll locking bug warning, indicating presence of 138badbc21a0. 
> Instead of saying to self "nope, just walk away", I poked SysRq-C.. and
> the bloody damn monitoring box received a 100% complete death rattle. 
> Well bugger.

Did you get a backtrace for this?

And to be clear, the steps to reproduce this are to boot a kernel with
lockdep enabled with netconsole on macb and then hit sysrq?

> Two trees contain locking buglet introducing 138badbc21a0, 6.12.41 and
> 6.17.0, but only the later reproduces the above mess.  A quick stare at
> git diff inspired checking e6a532185daa, and sure enough reverting only
> it reproduced a functional netconsole, modulo warnings, ie the mess
> above manifests only with BOTH resident.  Either revert 138badbc21a0,
> or revert e6a532185daa and fix the 138badbc21a0 locking buglet, and the
> result is the same, a perfectly functional warning free netconsole.