[PATCH 2/7] Revert "mm: vmscan: remove shrinker_rwsem from synchronize_shrinkers()"

[Qi Zheng]

From: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>

This reverts commit 1643db98d9b314e0a592d152603094fbf7ab906e.

Kernel test robot reports -88.8% regression in stress-ng.ramfs.ops_per_sec
test case [1], which is caused by commit f95bdb700bc6 ("mm: vmscan: make
global slab shrink lockless"). The root cause is that SRCU has to be careful
to not frequently check for SRCU read-side critical section exits. Therefore,
even if no one is currently in the SRCU read-side critical section,
synchronize_srcu() cannot return quickly. That's why unregister_shrinker()
has become slower.

We will try to use the refcount+RCU method [2] proposed by Dave Chinner
to continue to re-implement the lockless slab shrink. So we still need
shrinker_rwsem in synchronize_shrinkers() after reverting the shrinker_srcu
related changes.

[1]. https://lore.kernel.org/lkml/202305230837.db2c233f-yujie.liu@xxxxxxxxx/
[2]. https://lore.kernel.org/lkml/ZIJhou1d55d4H1s0@xxxxxxxxxxxxxxxxxxx/

Reported-by: kernel test robot <yujie.liu@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-lkp/202305230837.db2c233f-yujie.liu@xxxxxxxxx
Signed-off-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
---
 mm/vmscan.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4730dba253c8..0ba0e1180f3f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -831,11 +831,15 @@ EXPORT_SYMBOL(unregister_shrinker);
 /**
  * synchronize_shrinkers - Wait for all running shrinkers to complete.
  *
- * This is useful to guarantee that all shrinker invocations have seen an
- * update, before freeing memory.
+ * This is equivalent to calling unregister_shrink() and register_shrinker(),
+ * but atomically and with less overhead. This is useful to guarantee that all
+ * shrinker invocations have seen an update, before freeing memory, similar to
+ * rcu.
  */
 void synchronize_shrinkers(void)
 {
+	down_write(&shrinker_rwsem);
+	up_write(&shrinker_rwsem);
 	atomic_inc(&shrinker_srcu_generation);
 	synchronize_srcu(&shrinker_srcu);
 }
-- 
2.30.2