Re: [PATCH v2 1/1] hung_task: fix warnings caused by unaligned lock pointers

[Lance Yang <lance.yang@linux.dev>]

On 2025/10/8 15:09, Lance Yang wrote:
> 
> 
> On 2025/10/8 14:14, Finn Thain wrote:
>>
>> On Wed, 8 Oct 2025, Lance Yang wrote:
>>
>>> On 2025/10/8 08:40, Finn Thain wrote:
>>>>
>>>> On Tue, 7 Oct 2025, Andrew Morton wrote:
>>>>
>>>>> Getting back to the $Subject at hand, are people OK with proceeding
>>>>> with Lance's original fix?
>>>>>
>>>>
>>>> Lance's patch is probably more appropriate for -stable than the patch I
>>>> proposed -- assuming a fix is needed for -stable.
>>>
>>> Thanks!
>>>
>>> Apart from that, I believe this fix is still needed for the hung task
>>> detector itself, to prevent unnecessary warnings in a few unexpected
>>> cases.
>>>
>>
>> Can you be more specific about those cases? A fix for a theoretical bug
>> doesn't qualify for -stable branches. But if it's a fix for a real bug, I
>> have misunderstood Andrew's question...
> 
> I believe it is a real bug, as it was reported by Eero and Geert[1].
> 
> The blocker tracking mechanism in -stable assumes that lock pointers
> are at least 4-byte aligned. As I mentioned previously[2], this
> assumption fails for packed structs on architectures that don't trap
> on unaligned access.
> 
> Of course, we could always improve the mechanism to not make
> assumptions. But for -stable, this fix completely resolves the issue
> by ignoring any unaligned pointer, whatever the cause (e.g., packed
> structs, non-native alignment, etc.).
> 
> So we can all sleep well at night again :)
> 
> [1] https://lore.kernel.org/lkml/ 
> CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@mail.gmail.com/
> [2] https://lore.kernel.org/lkml/cfb62b9d-9cbd-47dd- 
> a894-3357027e2a50@linux.dev/

Forgot to add:

In other words, we are not just fixing the bug reported by Eero
and Geert, but correcting the blocker tracking mechanism's flawed
assumption for -stable ;)

If you feel this doesn't qualify as a fix, I can change the Fixes:
tag to point to the original commit that introduced this flawed
mechanism instead.

> 
>>
>>>>
>>>> Besides those two alternatives, there is also a workaround:
>>>> $ ./scripts/config -d DETECT_HUNG_TASK_BLOCKER
>>>> which may be acceptable to the interested parties (i.e. m68k users).
>>>>
>>>> I don't have a preference. I'll leave it up to the bug reporters (Eero
>>>> and Geert).
>>>
>