linux-kernel - Re: [PATCH v3 2/8] x86/mm: Change the leave

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Date:   Thu, 25 May 2017 19:01:40 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Rik van Riel <riel@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Nadav Amit <namit@...are.com>, Michal Hocko <mhocko@...e.com>,
        Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH v3 2/8] x86/mm: Change the leave_mm() condition for local
 TLB flushes

On Thu, May 25, 2017 at 6:39 PM, Rik van Riel <riel@...hat.com> wrote:
> On Thu, 2017-05-25 at 17:47 -0700, Andy Lutomirski wrote:
>>
>> +++ b/arch/x86/mm/tlb.c
>> @@ -311,7 +311,7 @@ void flush_tlb_mm_range(struct mm_struct *mm,
>> unsigned long start,
>>               goto out;
>>       }
>>
>> -     if (!current->mm) {
>> +     if (this_cpu_read(cpu_tlbstate.state) != TLBSTATE_OK) {
>>               leave_mm(smp_processor_id());
>
> Unless -mm changed leave_mm (I did not check), this
> is not quite correct yet.
>
> The reason is leave_mm (at least in the latest Linus
> tree) ignores the cpu argument for one of its checks.
>
> You should probably fix that in an earlier patch,
> assuming you haven't already done so in -mm.
>
> void leave_mm(int cpu)
> {
>         struct mm_struct *active_mm =
> this_cpu_read(cpu_tlbstate.active_mm);
>         if (this_cpu_read(cpu_tlbstate.state) == TLBSTATE_OK)
>                 BUG();
>         if (cpumask_test_cpu(cpu, mm_cpumask(active_mm))) {
>                 cpumask_clear_cpu(cpu, mm_cpumask(active_mm));
>                 load_cr3(swapper_pg_dir);

I agree it's odd, but what's the bug?  Both before and after, leave_mm
needed to be called with cpu == smp_processor_id(), and
smp_processor_id() warns if it's called in a preemptible context.