linux-kernel - [45/74] mm: prevent mmap_cache race in find

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Message-ID: <lsq.1365374742.143652390@decadent.org.uk>
Date:	Sun, 07 Apr 2013 23:45:42 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Hugh Dickins" <hughd@...gle.com>,
	"David Rientjes" <rientjes@...gle.com>,
	"Jan Stancek" <jstancek@...hat.com>
Subject: [45/74] mm: prevent mmap_cache race in find_vma()

3.2.43-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jan Stancek <jstancek@...hat.com>

commit b6a9b7f6b1f21735a7456d534dc0e68e61359d2c upstream.

find_vma() can be called by multiple threads with read lock
held on mm->mmap_sem and any of them can update mm->mmap_cache.
Prevent compiler from re-fetching mm->mmap_cache, because other
readers could update it in the meantime:

               thread 1                             thread 2
                                        |
  find_vma()                            |  find_vma()
    struct vm_area_struct *vma = NULL;  |
    vma = mm->mmap_cache;               |
    if (!(vma && vma->vm_end > addr     |
        && vma->vm_start <= addr)) {    |
                                        |    mm->mmap_cache = vma;
    return vma;                         |
     ^^ compiler may optimize this      |
        local variable out and re-read  |
        mm->mmap_cache                  |

This issue can be reproduced with gcc-4.8.0-1 on s390x by running
mallocstress testcase from LTP, which triggers:

  kernel BUG at mm/rmap.c:1088!
    Call Trace:
     ([<000003d100c57000>] 0x3d100c57000)
      [<000000000023a1c0>] do_wp_page+0x2fc/0xa88
      [<000000000023baae>] handle_pte_fault+0x41a/0xac8
      [<000000000023d832>] handle_mm_fault+0x17a/0x268
      [<000000000060507a>] do_protection_exception+0x1e2/0x394
      [<0000000000603a04>] pgm_check_handler+0x138/0x13c
      [<000003fffcf1f07a>] 0x3fffcf1f07a
    Last Breaking-Event-Address:
      [<000000000024755e>] page_add_new_anon_rmap+0xc2/0x168

Thanks to Jakub Jelinek for his insight on gcc and helping to
track this down.

Signed-off-by: Jan Stancek <jstancek@...hat.com>
Acked-by: David Rientjes <rientjes@...gle.com>
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
[bwh: Backported to 3.2: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 mm/mmap.c  |    2 +-
 mm/nommu.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1573,7 +1573,7 @@ struct vm_area_struct *find_vma(struct m
 	if (mm) {
 		/* Check the cache first. */
 		/* (Cache hit rate is typically around 35%.) */
-		vma = mm->mmap_cache;
+		vma = ACCESS_ONCE(mm->mmap_cache);
 		if (!(vma && vma->vm_end > addr && vma->vm_start <= addr)) {
 			struct rb_node * rb_node;
 
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -807,7 +807,7 @@ struct vm_area_struct *find_vma(struct m
 	struct vm_area_struct *vma;
 
 	/* check the cache first */
-	vma = mm->mmap_cache;
+	vma = ACCESS_ONCE(mm->mmap_cache);
 	if (vma && vma->vm_start <= addr && vma->vm_end > addr)
 		return vma;
 

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