linux-kernel - Re: [PATCH v2] x86/mm/tlb: Remove flush_tlb

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Date:   Fri, 26 Apr 2019 08:37:37 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH v2] x86/mm/tlb: Remove flush_tlb_info from the stack

> On Apr 26, 2019, at 12:53 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Thu, Apr 25, 2019 at 09:20:24PM +0000, Nadav Amit wrote:
> 
>>> I think that's a bug and PeterZ is fixing those.
>> 
>> This would be quite surprising.
> 
> I need to get back to that percpu series .... :/
> 
>> Even atomic_dec() does not imply a compilers
>> barrier. I think I should leave it as is for now, and let Peter change it
>> later if he decides to do so. Let me know if you disagree.
> 
> https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.kernel.org%2Fr%2F20190424124421.808471451%40infradead.org&amp;data=02%7C01%7Cnamit%40vmware.com%7Cc58182519059466b21e708d6ca1c5964%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636918620470835044&amp;sdata=PpijqcCDBmWdVcoPNtx6oiNJsXj%2FED1z5k%2BdUeCifJM%3D&amp;reserved=0

Interesting! (and thanks for the reference). Well, I said it would be quite
surprising, and I see you wrote the same thing in the patch ;-)

But correct me if I’m wrong - it does sound as if you “screw” all the uses
of atomic_inc() and atomic_dec() (~4000 instances) for the fewer uses of
smp_mb__after_atomic() and smp_mb__before_atomic() (~400 instances).

Do you intend to at least introduce a variant of atomic_inc() without a
memory barrier?