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SubjectRe: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping
> 
> Yes, system_state can roughly separate these callers of __create_pgd_mapping. When system_state > SYSTEM_BOOTING we can add the lock.
> Thus, I have the following change:
>
> static DEFINE_SPINLOCK(swapper_pgdir_lock);
> +static DEFINE_MUTEX(fixmap_lock);
>
> void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
> {
> @@ -329,6 +330,8 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
> }
> BUG_ON(p4d_bad(p4d));
>
> + if (system_state > SYSTEM_BOOTING)

As there is nothing smaller than SYSTEM_BOOTING, you can use
if (system_state != SYSTEM_BOOTING)

...

>
> It seems work and somehow simper. But I don't know if it is reasonable to do this. So, any idea? @Ard Biesheuvel @Catalin Marinas

It's worth looking at kernel/notifier.c, e.g.,
blocking_notifier_chain_register()

if (unlikely(system_state == SYSTEM_BOOTING))
return notifier_chain_register(&nh->head, n);

down_write(&nh->rwsem);
ret = notifier_chain_register(&nh->head, n);
up_write(&nh->rwsem);



If we decide to go down that path, we should make sure to add a comment like

/*
* No need for locking during early boot. And it doesn't work as
* expected with KASLR enabled where we might clear BSS twice.
*/

--
Thanks,

David / dhildenb

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