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

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Message-ID: <99410857-0e72-23e4-c60f-dea96427b85a@linux-m68k.org>
Date: Wed, 10 Sep 2025 10:07:04 +1000 (AEST)
From: Finn Thain <fthain@...ux-m68k.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
cc: Lance Yang <lance.yang@...ux.dev>, akpm@...ux-foundation.org, 
    amaindex@...look.com, anna.schumaker@...cle.com, boqun.feng@...il.com, 
    geert@...ux-m68k.org, 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, oak@...sinkinet.fi, 
    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 Tue, 9 Sep 2025, Kent Overstreet wrote:

> On Tue, Sep 09, 2025 at 10:52:43PM +0800, Lance Yang wrote:
> > From: Lance Yang <lance.yang@...ux.dev>
> > 
> > The blocker tracking mechanism assumes that lock pointers are at least
> > 4-byte aligned to use their lower bits for type encoding.
> > 
> > However, as reported by Eero Tamminen, some architectures like m68k
> > only guarantee 2-byte alignment of 32-bit values. This breaks the
> > assumption and causes two related WARN_ON_ONCE checks to trigger.
> 
> Isn't m68k the only architecture that's weird like this?
> 

No. Historically, Linux/CRIS did not naturally align integer types either. 
AFAIK, there's no standard that demands natural alignment of integer 
types. Linux ABIs differ significantly.

For example, Linux/i386 does not naturally align long longs. Therefore, 
x86 may be expected to become the next m68k (or CRIS) unless such 
assumptions are avoided and alignment requirements are made explicit.

The real problem here is the algorithm. Some under-resourced distros 
choose to blame the ABI instead of the algorithm, because in doing so, 
they are freed from having to work to improve upstream code bases.

IMHO, good C doesn't make alignment assumptions, because that hinders 
source code portability and reuse, as well as algorithm extensibility. 
We've seen it before. The issue here [1] is no different from the pointer 
abuse which we fixed in Cpython [2].

Linux is probably the only non-trivial program that could be feasibly 
rebuilt with -malign-int without ill effect (i.e. without breaking 
userland) but that sort of workaround would not address the root cause 
(i.e. algorithms with bad assumptions).

[1]
https://lore.kernel.org/lkml/CAMuHMdW7Ab13DdGs2acMQcix5ObJK0O2dG_Fxzr8_g58Rc1_0g@mail.gmail.com/

[2]
https://github.com/python/cpython/pull/135016