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

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Message-ID: <39f14032374d5d60c62b283637267a96ce535861.camel@gmx.de>
Date: Thu, 28 Aug 2025 19:26:54 +0200
From: Mike Galbraith <efault@....de>
To: Sean Anderson <sean.anderson@...ux.dev>, 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

On Thu, 2025-08-28 at 10:57 -0400, Sean Anderson wrote:
> 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?

Yes, logs for 6.12.41 and 6.17.0-rc2 attached.

Since a patch has meanwhile landed, also a log of patched 6.17.0-rc3
now gripe free (yay) but with aforementioned broken output, followed by
addition of the e6a532185daa revert to confirm it still cures that.

> 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?

Yup.

> > 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.
> 


View attachment "netconsole-rpi5-6.12-eth0.log" of type "text/x-log" (18613 bytes)

View attachment "netconsole-rpi5-6.17-eth0.log" of type "text/x-log" (17135 bytes)

View attachment "netconsole-rpi5-6.17-eth0+patch-and+revert.log" of type "text/x-log" (3698 bytes)