Revert "mm: srcu-ify shrinkers"

This reverts commit dab937da82f9504fbac75592a1825614318d73e7.

2 files changed, 25 insertions, 22 deletions

diff --git a/fs/super.c b/fs/super.c
index c917817c8d40..b61372354f2b 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -49,8 +49,8 @@ static char *sb_writers_name[SB_FREEZE_LEVELS] = {
  * One thing we have to be careful of with a per-sb shrinker is that we don't
  * drop the last active reference to the superblock from within the shrinker.
  * If that happens we could trigger unregistering the shrinker from within the
- * shrinker path. Hence we take a passive reference to the superblock to avoid
- * this from occurring.
+ * shrinker path and that leads to deadlock on the shrinker_rwsem. Hence we
+ * take a passive reference to the superblock to avoid this from occurring.
  */
 static unsigned long super_cache_scan(struct shrinker *shrink,
 				      struct shrink_control *sc)
@@ -121,8 +121,8 @@ static unsigned long super_cache_count(struct shrinker *shrink,
 	 * Don't call trylock_super as it is a potential
 	 * scalability bottleneck. The counts could get updated
 	 * between super_cache_count and super_cache_scan anyway.
-	 * SRCU guarantees object validity across this call -- thus
-	 * it is safe to call list_lru_shrink_count() and
+	 * Call to super_cache_count with shrinker_rwsem held
+	 * ensures the safety of call to list_lru_shrink_count() and
 	 * s_op->nr_cached_objects().
 	 */
 	if (sb->s_op && sb->s_op->nr_cached_objects)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4a860f0e37d7..6dffe70442a6 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -36,7 +36,7 @@
 #include <linux/cpuset.h>
 #include <linux/compaction.h>
 #include <linux/notifier.h>
-#include <linux/srcu.h>
+#include <linux/rwsem.h>
 #include <linux/delay.h>
 #include <linux/kthread.h>
 #include <linux/freezer.h>
@@ -146,9 +146,8 @@ int vm_swappiness = 60;
  */
 unsigned long vm_total_pages;
 
-DEFINE_STATIC_SRCU(shrinker_srcu);
 static LIST_HEAD(shrinker_list);
-static DEFINE_SPINLOCK(shrinker_list_lock);
+static DECLARE_RWSEM(shrinker_rwsem);
 
 #ifdef CONFIG_MEMCG
 static bool global_reclaim(struct scan_control *sc)
@@ -243,9 +242,9 @@ int register_shrinker(struct shrinker *shrinker)
 	if (!shrinker->nr_deferred)
 		return -ENOMEM;
 
-	spin_lock(&shrinker_list_lock);
-	list_add_tail_rcu(&shrinker->list, &shrinker_list);
-	spin_unlock(&shrinker_list_lock);
+	down_write(&shrinker_rwsem);
+	list_add_tail(&shrinker->list, &shrinker_list);
+	up_write(&shrinker_rwsem);
 	return 0;
 }
 EXPORT_SYMBOL(register_shrinker);
@@ -255,14 +254,9 @@ EXPORT_SYMBOL(register_shrinker);
  */
 void unregister_shrinker(struct shrinker *shrinker)
 {
-	spin_lock(&shrinker_list_lock);
-	list_del_rcu(&shrinker->list);
-	spin_unlock(&shrinker_list_lock);
-	/*
-	 * Before freeing nr_deferred, ensure all srcu
-	 * readers are done with their critical region.
-	 */
-	synchronize_srcu(&shrinker_srcu);
+	down_write(&shrinker_rwsem);
+	list_del(&shrinker->list);
+	up_write(&shrinker_rwsem);
 	kfree(shrinker->nr_deferred);
 }
 EXPORT_SYMBOL(unregister_shrinker);
@@ -414,7 +408,6 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 				 unsigned long nr_scanned,
 				 unsigned long nr_eligible)
 {
-	int idx;
 	struct shrinker *shrinker;
 	unsigned long freed = 0;
 
@@ -424,9 +417,18 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 	if (nr_scanned == 0)
 		nr_scanned = SWAP_CLUSTER_MAX;
 
-	idx = srcu_read_lock(&shrinker_srcu);
+	if (!down_read_trylock(&shrinker_rwsem)) {
+		/*
+		 * If we would return 0, our callers would understand that we
+		 * have nothing else to shrink and give up trying. By returning
+		 * 1 we keep it going and assume we'll be able to shrink next
+		 * time.
+		 */
+		freed = 1;
+		goto out;
+	}
 
-	list_for_each_entry_rcu(shrinker, &shrinker_list, list) {
+	list_for_each_entry(shrinker, &shrinker_list, list) {
 		struct shrink_control sc = {
 			.gfp_mask = gfp_mask,
 			.nid = nid,
@@ -442,7 +444,8 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
 		freed += do_shrink_slab(&sc, shrinker, nr_scanned, nr_eligible);
 	}
 
-	srcu_read_unlock(&shrinker_srcu, idx);
+	up_read(&shrinker_rwsem);
+out:
 	cond_resched();
 	return freed;
 }