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

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Message-ID: <d07778f8-8990-226b-5171-4a36e6e18f32@linux-m68k.org>
Date: Mon, 25 Aug 2025 20:49:38 +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


[Belated Cc linux-m68k...]

On Mon, 25 Aug 2025, Lance Yang wrote:

> 
> On 2025/8/25 14:17, Finn Thain wrote:
> > 
> > On Mon, 25 Aug 2025, Lance Yang wrote:
> > 
> >>
> >> What if we squash the runtime check fix into your patch?
> > 
> > Did my patch not solve the problem?
> 
> Hmm... it should solve the problem for natural alignment, which is a 
> critical fix.
> 
> But it cannot solve the problem of forced misalignment from drivers 
> using #pragma pack(1). The runtime warning will still trigger in those 
> cases.
> 
> I built a simple test module on a kernel with your patch applied:
> 
> ```
> #include <linux/module.h>
> #include <linux/init.h>
> 
> struct __attribute__((packed)) test_container {
>     char padding[49];
>     struct mutex io_lock;
> };
> 
> static int __init alignment_init(void)
> {
>     struct test_container cont;
>     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 15:44:50 2025] io_lock address offset mod 4: 1
> 

Thanks for sending code to illustrate your point. Unfortunately, I was not 
able to reproduce your results. Tested on x86, your test module shows no 
misalignment:

[131840.349042] io_lock address offset mod 4: 0

Tested on m68k I also get 0, given the patch at the top of this thread:

[    0.400000] io_lock address offset mod 4: 0

> 
> As we can see, a packed struct can still force the entire mutex object 
> to an unaligned address. With an address like this, the WARN_ON_ONCE can 
> still be triggered.

I don't think so. But there is something unexpected going on here -- the 
output from pahole appears to say io_lock is at offset 49, which seems to 
contradict the printk() output above.

struct test_container {
        char                       padding[49];          /*     0    49 */
        struct mutex               io_lock __attribute__((__aligned__(1))); /*    49    12 */

        /* size: 61, cachelines: 1, members: 2 */
        /* forced alignments: 1 */
        /* last cacheline: 61 bytes */
} __attribute__((__packed__));