linux-kernel - [git pull] x86: fix global_flush

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Date:	Fri, 19 Oct 2007 12:48:55 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Greg KH <gregkh@...e.de>,
	Chris Wright <chrisw@...s-sol.org>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Beulich <jbeulich@...ell.com>, linux-kernel@...r.kernel.org
Subject: [git pull] x86: fix global_flush_tlb() bug


find a fix for a pretty serious global_flush_tlb() x86-64 bug below, 
-stable candidate too i think.

Linus, please pull this fix from the x86 git tree:

  ssh://master.kernel.org/pub/scm/linux/kernel/git/tglx/linux-2.6-x86.git
  |
  |  Ingo Molnar (1):
  |           x86: fix global_flush_tlb() bug

thanks,

	Ingo

------------------>
Subject: x86: fix global_flush_tlb() bug
From: Ingo Molnar <mingo@...e.hu>

While we were reviewing pageattr_32/64.c for unification,
Thomas Gleixner noticed the following serious SMP bug in
global_flush_tlb():

	down_read(&init_mm.mmap_sem);
	list_replace_init(&deferred_pages, &l);
	up_read(&init_mm.mmap_sem);

this is SMP-unsafe because list_replace_init() done on two CPUs in
parallel can corrupt the list.

This bug has been introduced about a year ago in the 64-bit tree:

       commit ea7322decb974a4a3e804f96a0201e893ff88ce3
       Author: Andi Kleen <ak@...e.de>
       Date:   Thu Dec 7 02:14:05 2006 +0100

       [PATCH] x86-64: Speed and clean up cache flushing in change_page_attr

                down_read(&init_mm.mmap_sem);
        -       dpage = xchg(&deferred_pages, NULL);
        +       list_replace_init(&deferred_pages, &l);
                up_read(&init_mm.mmap_sem);

the xchg() based version was SMP-safe, but list_replace_init() is not.
So this "cleanup" introduced a nasty bug.

why this bug never become prominent is a mystery - it can probably be
explained with the (still) relative obscurity of the x86_64 architecture.

the safe fix for now is to write-lock init_mm.mmap_sem.

Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 arch/x86/mm/pageattr_64.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

Index: linux/arch/x86/mm/pageattr_64.c
===================================================================
--- linux.orig/arch/x86/mm/pageattr_64.c
+++ linux/arch/x86/mm/pageattr_64.c
@@ -255,9 +255,14 @@ void global_flush_tlb(void)
 	struct page *pg, *next;
 	struct list_head l;
 
-	down_read(&init_mm.mmap_sem);
+	/*
+	 * Write-protect the semaphore, to exclude two contexts
+	 * doing a list_replace_init() call in parallel and to
+	 * exclude new additions to the deferred_pages list:
+	 */
+	down_write(&init_mm.mmap_sem);
 	list_replace_init(&deferred_pages, &l);
-	up_read(&init_mm.mmap_sem);
+	up_write(&init_mm.mmap_sem);
 
 	flush_map(&l);
 
-
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