Re: [PATCH V3] riscv: asid: Fixup stale TLB entry cause application crash

[Sergey Matyukevich]

Hi Guo Ren,


> After use_asid_allocator is enabled, the userspace application will
> crash by stale TLB entries. Because only using cpumask_clear_cpu without
> local_flush_tlb_all couldn't guarantee CPU's TLB entries were fresh.
> Then set_mm_asid would cause the user space application to get a stale
> value by stale TLB entry, but set_mm_noasid is okay.

... [snip]

> +	/*
> +	 * The mm_cpumask indicates which harts' TLBs contain the virtual
> +	 * address mapping of the mm. Compared to noasid, using asid
> +	 * can't guarantee that stale TLB entries are invalidated because
> +	 * the asid mechanism wouldn't flush TLB for every switch_mm for
> +	 * performance. So when using asid, keep all CPUs footmarks in
> +	 * cpumask() until mm reset.
> +	 */
> +	cpumask_set_cpu(cpu, mm_cpumask(next));
> +	if (static_branch_unlikely(&use_asid_allocator)) {
> +		set_mm_asid(next, cpu);
> +	} else {
> +		cpumask_clear_cpu(cpu, mm_cpumask(prev));
> +		set_mm_noasid(next);
> +	}
>  }

I observe similar user-space crashes on my SMP systems with enabled ASID.
My attempt to fix the issue was a bit different, see the following patch:

https://lore.kernel.org/linux-riscv/20220829205219.283543-1-geomatsi@xxxxxxxxx/

In brief, the idea was borrowed from flush_icache_mm handling:
- keep track of CPUs not running the task
- perform per-ASID TLB flush on such CPUs only if the task is switched there

Your patch also works fine in my tests fixing those crashes. I have a
question though, regarding removed cpumask_clear_cpu. How CPUs no more
running the task are removed from its mm_cpumask ? If they are not
removed, then flush_tlb_mm/flush_tlb_page will broadcast unnecessary
TLB flushes to those CPUs when ASID is enabled.

Regards,
Sergey