Re: [PATCH v2 01/11] asm-generic/tlb: Fix MMU_GATHER_TABLE_FREE

Will Deacon
2020-09-21T12:02:06Z

On Fri, Jul 17, 2020 at 01:10:06PM +0200, Peter Zijlstra wrote:
> The first MMU_GATHER_TABLE_FREE user showed a logic error in the
> tlb_needs_table_invalidate() definition. Make sure any TABLE_FREE has
> it defined.

Could you elaborate on the logic error, please? It's difficult to see
through all of the #ifdefs, but afaict we #error if
tlb_needs_table_invalidate() is defined but not CONFIG_MMU_GATHER_RCU_TABLE_FREE
with and without this patch. In other words, I'm failing to see what this
patch changes!

> Fixes: 0d6e24d430ef ("asm-generic/tlb: provide MMU_GATHER_TABLE_FREE")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> include/asm-generic/tlb.h | 35 +++++++++++++++++++----------------
> 1 file changed, 19 insertions(+), 16 deletions(-)
>
> --- a/include/asm-generic/tlb.h
> +++ b/include/asm-generic/tlb.h
> @@ -172,6 +172,18 @@
> * various ptep_get_and_clear() functions.
> */
>
> +#ifndef CONFIG_MMU_GATHER_RCU_TABLE_FREE
> +
> +/*
> + * Only RCU table free can override this; otherwise the TLBI is needed to
> + * provide existence guarantees for software walkers.
> + */
> +#ifdef tlb_needs_table_invalidate
> +#error tlb_needs_table_invalidate() requires MMU_GATHER_RCU_TABLE_FREE
> +#endif
> +
> +#endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
> +
> #ifdef CONFIG_MMU_GATHER_TABLE_FREE
>
> struct mmu_table_batch {
> @@ -187,17 +199,6 @@ struct mmu_table_batch {
>
> extern void tlb_remove_table(struct mmu_gather *tlb, void *table);
>
> -#else /* !CONFIG_MMU_GATHER_HAVE_TABLE_FREE */
> -
> -/*
> - * Without MMU_GATHER_TABLE_FREE the architecture is assumed to have page based
> - * page directories and we can use the normal page batching to free them.
> - */
> -#define tlb_remove_table(tlb, page) tlb_remove_page((tlb), (page))
> -
> -#endif /* CONFIG_MMU_GATHER_TABLE_FREE */
> -
> -#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
> /*
> * This allows an architecture that does not use the linux page-tables for
> * hardware to skip the TLBI when freeing page tables.
> @@ -206,13 +207,15 @@ extern void tlb_remove_table(struct mmu_
> #define tlb_needs_table_invalidate() (true)
> #endif
>
> -#else
> +#else /* !CONFIG_MMU_GATHER_HAVE_TABLE_FREE */

While you're at it, this comment can be fixed to refer to
CONFIG_MMU_GATHER_RCU_TABLE_FREE.

Will