Re: [PATCH] device property: Make modifications of fwnode "flags" thread safe

[Geert Uytterhoeven]

Hi Andy,

On Tue, 17 Mar 2026 at 08:34, Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
> On Tue, Mar 17, 2026 at 08:26:53AM +0100, Wolfram Sang wrote:
>
> ...
>
> > Thanks for tackling this issue! I agree it should be fixed, just
> > wondered about one thing:
> >
> > > While flags are often modified while under the "fwnode_link_lock",
> > > this is not universally true.
> >
> > Is it a possibility to use the lock in all code paths instead?
> > Because...
> >
> > >     struct list_head consumers;
> > > -   u8 flags;
> > > +   unsigned long flags;
> >
> > ... this change costs some memory on every system. Maybe it can be
> > avoided?
>
> How much memory does it cost? On most 64-bit architectures is +4 bytes,
> rarely +0 bytes, on m68k it might be +2bytes. On 32-bit it most likely
> +0 bytes. I expect that 64-bit machines will cope with this bump.

On all architectures with natural alignment of pointers and longs,
it won't cost a thing: struct list_head contains pointers, so the
struct must be padded to a multiple of 4 or 8 bytes anyway.
On m68k[*],  it will cost 2 bytes, as the existing padding is just a
single byte.

[*] Iff m68k ever switches to 32-bit alignment, there won't be an
    additional cost due to the change of flags here, but of course
    there would be a cost all over the place.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds