Re: [PATCH] atomic: Specify natural alignment for atomic_t

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

On 2025/8/28 07:43, Finn Thain wrote:
> 
> 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.

Thanks for taking the time to reproduce it!

This proves the problem can happen in practice (e.g., with packed structs),
so we need to ignore the unaligned pointers on the architectures that don't
trap for now.

Cheers,
Lance