[PATCH] slab: fix an infinite loop in leaks_show()

[Qian Cai]

"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

Fixes: d31676dfde25 ("mm/slab: alternative implementation for DEBUG_SLAB_LEAK")
Signed-off-by: Qian Cai <cai@xxxxxx>
---
 mm/slab.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/slab.c b/mm/slab.c
index 9142ee992493..3e1b7ff0360c 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -4328,8 +4328,12 @@ 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 always make kmemleak_object and
+		 * debug_objects_cache dirty, so reset afterwards.
+		 */
+		set_store_user_clean(cachep);
 
 		x[1] = 0;
 
-- 
2.17.2 (Apple Git-113)