Re: [PATCH RFC] m68k: skip kernel premption if interrupts were disabled

Michael Schmitz
2024-03-26T19:59:17Z

Hi Finn,

thanks - I'm pretty sure tried that early on but botched it by excessive
locking (i.e., keeping preemption disabled when calling
get_zeroed_page()!).

So I don't think you do use too much locking here.

I'm running stress tests for a while now, without any trouble so far.
Need to add a few other stressors back in, and repeat all that on a
slower ARAnyM instance but I'm quite confident you found the solution.

Geert: with this data race fixed, it does appear my RFC patch is no
longer needed. Finn or I probably ought to prepare a new RFC patch to go
on top of your preemption patch. There is no commit ID to use in a
Fixes: tag for that one, correct?

Cheers,

Michael

On 26/03/24 22:58, Finn Thain wrote:
> On Tue, 26 Mar 2024, Michael Schmitz wrote:
>
>> see below (hopefully not whitespace-damaged).
>>
> Following your approach, I added mutual exclusion to the other candidate
> in the same file, namely get_pointer_table(). Together with your patch,
> that seems to be sufficient to make stress-ng happy. I don't know the real
> extent of the data race here so I've probably used too much locking in
> this experiment (?)
>
> diff --git a/arch/m68k/mm/motorola.c b/arch/m68k/mm/motorola.c
> index 068ad7c9bb5c..d1ad55ee1b60 100644
> --- a/arch/m68k/mm/motorola.c
> +++ b/arch/m68k/mm/motorola.c
> @@ -140,10 +140,14 @@ void __init init_pointer_table(void *table, int type)
>
> void *get_pointer_table(int type)
> {
> - ptable_desc *dp = ptable_list[type].next;
> - unsigned int mask = list_empty(&ptable_list[type]) ? 0 : PD_MARKBITS(dp);
> + ptable_desc *dp;
> + unsigned int mask;
> unsigned int tmp, off;
>
> + preempt_disable();
> +
> + dp = ptable_list[type].next;
> + mask = list_empty(&ptable_list[type]) ? 0 : PD_MARKBITS(dp);
> /*
> * For a pointer table for a user process address space, a
> * table is taken from a page allocated for the purpose. Each
> @@ -154,8 +158,13 @@ void *get_pointer_table(int type)
> void *page;
> ptable_desc *new;
>
> - if (!(page = (void *)get_zeroed_page(GFP_KERNEL)))
> + sched_preempt_enable_no_resched();
> +
> + if (!(page = (void *)get_zeroed_page(GFP_KERNEL))) {
> return NULL;
> + }
> +
> + preempt_disable();
>
> if (type == TABLE_PTE) {
> /*
> @@ -171,6 +180,7 @@ void *get_pointer_table(int type)
> PD_MARKBITS(new) = ptable_mask(type) - 1;
> list_add_tail(new, dp);
>
> + sched_preempt_enable_no_resched();
> return (pmd_t *)page;
> }
>
> @@ -181,6 +191,7 @@ void *get_pointer_table(int type)
> /* move to end of list */
> list_move_tail(dp, &ptable_list[type]);
> }
> + sched_preempt_enable_no_resched();
> return page_address(PD_PAGE(dp)) + off;
> }
>