Re: [PATCH 1/4] arm64: mm: Fix TLBI vs ASID rollover

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Aug 06, 2021 at 12:31:04PM +0100, Will Deacon wrote:
> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> index 75beffe2ee8a..e9c30859f80c 100644
> --- a/arch/arm64/include/asm/mmu.h
> +++ b/arch/arm64/include/asm/mmu.h
> @@ -27,11 +27,32 @@ typedef struct {
>  } mm_context_t;
>  
>  /*
> - * This macro is only used by the TLBI and low-level switch_mm() code,
> - * neither of which can race with an ASID change. We therefore don't
> - * need to reload the counter using atomic64_read().
> + * We use atomic64_read() here because the ASID for an 'mm_struct' can
> + * be reallocated when scheduling one of its threads following a
> + * rollover event (see new_context() and flush_context()). In this case,
> + * a concurrent TLBI (e.g. via try_to_unmap_one() and ptep_clear_flush())
> + * may use a stale ASID. This is fine in principle as the new ASID is
> + * guaranteed to be clean in the TLB, but the TLBI routines have to take
> + * care to handle the following race:
> + *
> + *    CPU 0                    CPU 1                          CPU 2
> + *
> + *    // ptep_clear_flush(mm)
> + *    xchg_relaxed(pte, 0)
> + *    DSB ISHST
> + *    old = ASID(mm)

We'd need specs clarified (ARM ARM, cat model) that the DSB ISHST is
sufficient to order the pte write with the subsequent ASID read.
Otherwise the patch looks fine to me:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>