linux-kernel - [PATCH AUTOSEL 5.0 020/173] mm/slab.c: fix an infinite loop in leaks

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Message-Id: <20190601131934.25053-20-sashal@kernel.org>
Date:   Sat,  1 Jun 2019 09:16:52 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Qian Cai <cai@....pw>, Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>, linux-mm@...ck.org
Subject: [PATCH AUTOSEL 5.0 020/173] mm/slab.c: fix an infinite loop in leaks_show()

From: Qian Cai <cai@....pw>

[ Upstream commit 745e10146c31b1c6ed3326286704ae251b17f663 ]

"cat /proc/slab_allocators" could hang forever on SMP machines with
kmemleak or object debugging enabled due to other CPUs running do_drain()
will keep making kmemleak_object or debug_objects_cache dirty and unable
to escape the first loop in leaks_show(),

do {
	set_store_user_clean(cachep);
	drain_cpu_caches(cachep);
	...

} while (!is_store_user_clean(cachep));

For example,

do_drain
  slabs_destroy
    slab_destroy
      kmem_cache_free
        __cache_free
          ___cache_free
            kmemleak_free_recursive
              delete_object_full
                __delete_object
                  put_object
                    free_object_rcu
                      kmem_cache_free
                        cache_free_debugcheck --> dirty kmemleak_object

One approach is to check cachep->name and skip both kmemleak_object and
debug_objects_cache in leaks_show().  The other is to set store_user_clean
after drain_cpu_caches() which leaves a small window between
drain_cpu_caches() and set_store_user_clean() where per-CPU caches could
be dirty again lead to slightly wrong information has been stored but
could also speed up things significantly which sounds like a good
compromise.  For example,

 # cat /proc/slab_allocators
 0m42.778s # 1st approach
 0m0.737s  # 2nd approach

[akpm@...ux-foundation.org: tweak comment]
Link: http://lkml.kernel.org/r/20190411032635.10325-1-cai@lca.pw
Fixes: d31676dfde25 ("mm/slab: alternative implementation for DEBUG_SLAB_LEAK")
Signed-off-by: Qian Cai <cai@....pw>
Reviewed-by: Andrew Morton <akpm@...ux-foundation.org>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Pekka Enberg <penberg@...nel.org>
Cc: David Rientjes <rientjes@...gle.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 mm/slab.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/slab.c b/mm/slab.c
index f4bbc53008f3b..932a439149cdb 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -4317,8 +4317,12 @@ static int leaks_show(struct seq_file *m, void *p)
 	 * whole processing.
 	 */
 	do {
-		set_store_user_clean(cachep);
 		drain_cpu_caches(cachep);
+		/*
+		 * drain_cpu_caches() could make kmemleak_object and
+		 * debug_objects_cache dirty, so reset afterwards.
+		 */
+		set_store_user_clean(cachep);
 
 		x[1] = 0;
 
-- 
2.20.1