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.