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Subject[PATCH AUTOSEL 5.0 020/173] mm/slab.c: fix an infinite loop in leaks_show()
Date
From: Qian Cai <cai@lca.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@linux-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@lca.pw>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.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
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