Re: [PATCH v3 04/49] mm: shrinker: remove redundant shrinker_rwsem in debugfs operations

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Hi Simon,

On 2023/7/28 16:13, Simon Horman wrote:
On Thu, Jul 27, 2023 at 04:04:17PM +0800, Qi Zheng wrote:
The debugfs_remove_recursive() will wait for debugfs_file_put() to return,
so the shrinker will not be freed when doing debugfs operations (such as
shrinker_debugfs_count_show() and shrinker_debugfs_scan_write()), so there
is no need to hold shrinker_rwsem during debugfs operations.

Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
Reviewed-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
  mm/shrinker_debug.c | 14 --------------
  1 file changed, 14 deletions(-)

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index 3ab53fad8876..f1becfd45853 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -55,11 +55,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
  	if (!count_per_node)
  		return -ENOMEM;
- ret = down_read_killable(&shrinker_rwsem);
-	if (ret) {
-		kfree(count_per_node);
-		return ret;
-	}
  	rcu_read_lock();

Hi Qi Zheng,

As can be seen in the next hunk, this function returns 'ret'.
However, with this change 'ret' is uninitialised unless
signal_pending() returns non-zero in the while loop below.

Thanks for your feedback, the 'ret' should be initialized to 0,
will fix it.

Thanks,
Qi


This is flagged in a clan-16 W=1 build.

  mm/shrinker_debug.c:87:11: warning: variable 'ret' is used uninitialized whenever 'do' loop exits because its condition is false [-Wsometimes-uninitialized]
          } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  mm/shrinker_debug.c:92:9: note: uninitialized use occurs here
          return ret;
                 ^~~
  mm/shrinker_debug.c:87:11: note: remove the condition if it is always true
          } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                   1
  mm/shrinker_debug.c:77:7: warning: variable 'ret' is used uninitialized whenever 'if' condition is true [-Wsometimes-uninitialized]
                  if (!memcg_aware) {
                      ^~~~~~~~~~~~
  mm/shrinker_debug.c:92:9: note: uninitialized use occurs here
          return ret;
                 ^~~
  mm/shrinker_debug.c:77:3: note: remove the 'if' if its condition is always false
                  if (!memcg_aware) {
                  ^~~~~~~~~~~~~~~~~~~
  mm/shrinker_debug.c:52:9: note: initialize the variable 'ret' to silence this warning
          int ret, nid;
                 ^
                  = 0

memcg_aware = shrinker->flags & SHRINKER_MEMCG_AWARE;
@@ -92,7 +87,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
  	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
rcu_read_unlock();
-	up_read(&shrinker_rwsem);
kfree(count_per_node);
  	return ret;

...

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