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

[David Laight <david.laight.linux@gmail.com>]

On Mon, 25 Aug 2025 15:46:42 +0800
Lance Yang <lance.yang@linux.dev> 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);

Doesn't that give a compilation warning from 'taking the address of a packed member'?
Ignore that at your peril.

More problematic is that, IIRC, m68k kmalloc() allocates 16bit aligned memory.
This has broken other things in the past.
I doubt that increasing the alignment to 32bits would make much difference
to the kernel memory footprint.

	David


>      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
> 
> 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.
> 
> That's why I proposed squashing the runtime check fix into your patch.
> Then it can be cleanly backported to stop all the spurious warnings at
> once.
> 
> I hope this clarifies things.
> 
>