linux-kernel - Re: [PATCH] atomic: Specify natural alignment for atomic

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Message-ID: <4405ee5a-becc-7375-61a9-01304b3e0b20@linux-m68k.org>
Date: Thu, 28 Aug 2025 09:43:49 +1000 (AEST)
From: Finn Thain <fthain@...ux-m68k.org>
To: Lance Yang <lance.yang@...ux.dev>
cc: akpm@...ux-foundation.org, geert@...ux-m68k.org, 
    linux-kernel@...r.kernel.org, mhiramat@...nel.org, oak@...sinkinet.fi, 
    peterz@...radead.org, stable@...r.kernel.org, will@...nel.org, 
    Lance Yang <ioworker0@...il.com>, linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH] atomic: Specify natural alignment for atomic_t


On Mon, 25 Aug 2025, Lance Yang wrote:

> 
> Same here, using a global static variable instead of a local one. The 
> result is consistently misaligned.
> 
> ```
> #include <linux/module.h>
> #include <linux/init.h>
> 
> static struct __attribute__((packed)) test_container {
>     char padding[49];
>     struct mutex io_lock;
> } cont;
> 
> static int __init alignment_init(void)
> {
>     pr_info("Container base address      : %px\n", &cont);
>     pr_info("io_lock member address      : %px\n", &cont.io_lock);
>     pr_info("io_lock address offset mod 4: %lu\n", (unsigned long)&cont.io_lock % 4);
>     return 0;
> }
> 
> static void __exit alignment_exit(void)
> {
>     pr_info("Module unloaded\n");
> }
> 
> module_init(alignment_init);
> module_exit(alignment_exit);
> MODULE_LICENSE("GPL");
> MODULE_AUTHOR("x");
> MODULE_DESCRIPTION("x");
> ```
> 
> Result from dmesg:
> 
> ```
> [Mon Aug 25 19:33:28 2025] Container base address      : ffffffffc28f0940
> [Mon Aug 25 19:33:28 2025] io_lock member address      : ffffffffc28f0971
> [Mon Aug 25 19:33:28 2025] io_lock address offset mod 4: 1
> ```
> 

FTR, I was able to reproduce that result (i.e. static storage):

[    0.320000] Container base address      : 0055d9d0
[    0.320000] io_lock member address      : 0055da01
[    0.320000] io_lock address offset mod 4: 1

I think the experiments you sent previously would have demonstrated the 
same result, except for the unpredictable base address that you sensibly 
logged in this version.