linux-kernel - Re: [RFC 05/13] x86/mm: Add barriers and document switch

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Date:	Thu, 9 Jun 2016 12:45:55 -0700
From:	Nadav Amit <nadav.amit@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Brian Gerst <brgerst@...il.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [RFC 05/13] x86/mm: Add barriers and document switch_mm-vs-flush synchronization

Andy Lutomirski <luto@...capital.net> wrote:

> On Fri, Jun 3, 2016 at 10:42 AM, Nadav Amit <nadav.amit@...il.com> wrote:
>> Following this patch, if (current->active_mm != mm), flush_tlb_page() still
>> doesn’t call smp_mb() before checking mm_cpumask(mm).
>> 
>> In contrast, flush_tlb_mm_range() does call smp_mb().
>> 
>> Is there a reason for this discrepancy?
> 
> Not that I can remember.  Is the remote flush case likely to be racy?

You replied separately on another email that included a patch to fix
this case. It turns out smp_mb is not needed on flush_tlb_page, since
the PTE is always updated using an atomic operation. Yet, a compiler 
barrier is still needed, so I added smp_mb__after_atomic instead.

Nadav