linux-kernel - Re: [PATCH 02/20] mm: Add optional TLB flush to generic RCU page-table freeing

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Date:	Wed, 27 Jun 2012 16:42:52 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Alex Shi <alex.shi@...el.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Russell King <rmk@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Tony Luck <tony.luck@...el.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Kyle McMartin <kyle@...artin.ca>,
	James Bottomley <jejb@...isc-linux.org>,
	Chris Zankel <chris@...kel.net>
Subject: Re: [PATCH 02/20] mm: Add optional TLB flush to generic RCU
 page-table freeing

On Wed, Jun 27, 2012 at 4:01 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>
> How about something like this

Looks better.

I'd be even happier if you made the whole

  "When there's less then two users.."

(There's a misspelling there, btw, I didn't notice until I
cut-and-pasted that) logic be a helper function, and have that helper
function be inside that same #ifdef CONFIG_STRICT_TLB_FILL block
together witht he tlb_table_flush_mmu() function.

IOW, something like

  static int tlb_remove_table_quick( struct mmu_gather *tlb, void *table)
  {
        if (atomic_read(&tlb->mm->mm_users) < 2) {
            __tlb_remove_table(table);
            return 1;
        }
        return 0;
  }

for the CONFIG_STRICT_TLB_FILL case, and then the default case just
does an unconditional "return 0".

So that the actual code can avoid having #ifdef's in the middle of a
function, and could just do

    if (tlb_remove_table_quick(tlb, table))
        return;

instead.

Maybe it's just me, but I detest seeing #ifdef's in the middle of
code. I'd much rather have the #ifdef's *outside* the code and have
these kinds of helper functions that sometimes end up becoming empty.

                   Linus
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