[PATCH v4 03/12] ARM: vfp: Fix broken softirq handling with instrumentation enabled

[Guenter Roeck]

On 4/9/23 07:29, Linux regression tracking (Thorsten Leemhuis) wrote:
> On 20.03.23 14:18, Ard Biesheuvel wrote:
>> Commit 62b95a7b44d1 ("ARM: 9282/1: vfp: Manipulate task VFP state with
>> softirqs disabled") replaced the en/disable preemption calls inside the
>> VFP state handling code with en/disabling of soft IRQs, which is
>> necessary to allow kernel use of the VFP/SIMD unit when handling a soft
>> IRQ.
>>
>> Unfortunately, when lockdep is enabled (or other instrumentation that
>> enables TRACE_IRQFLAGS), the disable path implemented in asm fails to
>> perform the lockdep and RCU related bookkeeping, resulting in spurious
>> warnings and other badness.
>>
>> Set let's rework the VFP entry code a little bit so we can make the
>> local_bh_disable() call from C, with all the instrumentations that
>> happen to have been configured. Calling local_bh_enable() can be done
>> from asm, as it is a simple wrapper around __local_bh_enable_ip(), which
>> is always a callable function.
>>
>> Link: https://lore.kernel.org/all/ZBBYCSZUJOWBg1s8@localhost.localdomain/
>> Fixes: 62b95a7b44d1 ("ARM: 9282/1: vfp: Manipulate task VFP state with softirqs disabled")
>> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
>> Reviewed-by: Linus Walleij <linus.walleij at linaro.org>
>> Tested-by: Guenter Roeck <linux at roeck-us.net>
>> [...]
> 
> It feels a lot like I'm missing something, but I can't figure out what,
> I thus have to ask:
> 
> What happened to this patch and/or the series? From my very limited
> viewpoint it looks like there was no progress for nearly three week now.
> 
> Reminder, 62b95a7b44d1 is a commit from 6.3 cycle and fixing a problem
> Guenter ran into. Hence it to me looks like it's something that better
> would be fixed this cycle. Or is this considered too dangerous for some
> reason and thus need to wait? But even then I wonder a little bit why
> it's not in next by now.
> 

Something for sure has changed. I can no longer reproduce the problem
with the latest kernel (v6.3-rc6). I can still reproduce it with
v6.3-rc2. Further testing shows that it is no longer reproducible
for me with v6.3-rc3. That made me curious. I am currently bisecting,
but that will take a while.

Guenter