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

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Message-ID: <ad7cb710-0d5a-93b1-fa4d-efb236760495@linux-m68k.org>
Date: Thu, 9 Oct 2025 08:55:21 +1100 (AEDT)
From: Finn Thain <fthain@...ux-m68k.org>
To: Lance Yang <lance.yang@...ux.dev>
cc: Andrew Morton <akpm@...ux-foundation.org>, 
    Geert Uytterhoeven <geert@...ux-m68k.org>, 
    Eero Tamminen <oak@...sinkinet.fi>, 
    Kent Overstreet <kent.overstreet@...ux.dev>, amaindex@...look.com, 
    anna.schumaker@...cle.com, boqun.feng@...il.com, ioworker0@...il.com, 
    joel.granados@...nel.org, jstultz@...gle.com, leonylgao@...cent.com, 
    linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org, 
    longman@...hat.com, mhiramat@...nel.org, mingo@...hat.com, 
    mingzhe.yang@...com, peterz@...radead.org, rostedt@...dmis.org, 
    senozhatsky@...omium.org, tfiga@...omium.org, will@...nel.org, 
    stable@...r.kernel.org
Subject: Re: [PATCH v2 1/1] hung_task: fix warnings caused by unaligned lock
 pointers


On Wed, 8 Oct 2025, Lance Yang wrote:

> On 2025/10/8 18:12, Finn Thain wrote:
> > 
> > On Wed, 8 Oct 2025, Lance Yang wrote:
> > 
> >>
> >> 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.
> >>
> > 
> > That's really a question for the bug reporters. I don't personally 
> > have a problem with CONFIG_DETECT_HUNG_TASK_BLOCKER so I can't say 
> > whether the fix meets the requirements set in 
> > Documentation/process/stable-kernel-rules.rst. And I still don't know
> 
> I'm a bit confused, as I recall you previously stating that "It's wrong 
> and should be fixed"[1].
> 

You took that quote out of context. Please go and read it again.

> To clarify, is your current position that it should be fixed in general, 
> but the fix should not be backported to -stable?
> 

To clarify, what do you mean by "it"? Is it the commentary discussed in 
[1]? The misalignment of atomics? The misalignment of locks? The alignment 
assumptions in your code? The WARN reported by Eero and Geert?

> If so, then I have nothing further to add to this thread and am happy to 
> let the maintainer @Andrew decide.
> 
> > what's meant by "unnecessary warnings in a few unexpected cases".
> 
> The blocker tracking mechanism will trigger a warning when it encounters 
> any unaligned lock pointer (e.g., from a packed struct). I don't think 
> that is the expected behavior.

Sure, no-one was expecting false positives.

I think you are conflating "misaligned" with "not 4-byte aligned". Your 
algorithm does not strictly require natural alignment, it requires 4-byte 
alignment of locks.

Regarding your concern about packed structs, please re-read this message: 
https://lore.kernel.org/all/CAMuHMdV-AtPm-W-QUC1HixJ8Koy_HdESwCCOhRs3Q26=wjWwog@mail.gmail.com/

AFAIK the problem with your code is nothing more than the usual difficulty 
encountered when porting between architectures that have different 
alignment rules for scalar variables.

Therefore, my question about the theoretical nature of the problem comes 
down to this.

Is the m68k architecture the only one producing actual false positives? 

Do you know of actual instances of locks in packed structs?

> Instead, it should simply skip any unaligned pointer it cannot handle. 
> For the stable kernels, at least, this is the correct behavior.
> 

Why? Are users of the stable branch actually affected?

> [1]
> https://lore.kernel.org/lkml/6ec95c3f-365b-e352-301b-94ab3d8af73c@linux-m68k.org/
> 
>