linux-kernel - [PATCH 5/7] kernfs: Make kernfs_drain() skip draining more aggressively

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Date:   Fri, 19 Aug 2022 14:05:49 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Imran Khan <imran.f.khan@...cle.com>, kernel-team@...com,
        Tejun Heo <tj@...nel.org>
Subject: [PATCH 5/7] kernfs: Make kernfs_drain() skip draining more aggressively

kernfs_drain() is updated to handle whether draining is necessary or not
rather than its caller. __kernfs_remove() now always calls kernfs_drain()
instead of filtering based on KERNFS_ACTIVATED.

kernfs_drain() now tests kn->active and kernfs_should_drain_open_files() to
determine whether draining is necessary at all. If not, it returns %false
without doing anything. Otherwise, it unlocks kernfs_rwsem and drains as
before and returns %true. The return value isn't used yet.

Using the precise conditions allows skipping more aggressively. This isn't a
meaningful optimization on its own but will enable future stand-alone
kernfs_deactivate() implementation.

While at it, make minor comment updates.

Signed-off-by: Tejun Heo <tj@...nel.org>
---
 fs/kernfs/dir.c | 34 ++++++++++++++++++++--------------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 8ae44db920d4..f857731598cd 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -460,10 +460,14 @@ void kernfs_put_active(struct kernfs_node *kn)
  * @kn: kernfs_node to drain
  *
  * Drain existing usages and nuke all existing mmaps of @kn.  Mutiple
- * removers may invoke this function concurrently on @kn and all will
+ * callers may invoke this function concurrently on @kn and all will
  * return after draining is complete.
+ *
+ * RETURNS:
+ * %false if nothing needed to be drained; otherwise, %true. On %true return,
+ * kernfs_rwsem has been released and re-acquired.
  */
-static void kernfs_drain(struct kernfs_node *kn)
+static bool kernfs_drain(struct kernfs_node *kn)
 	__releases(&kernfs_root(kn)->kernfs_rwsem)
 	__acquires(&kernfs_root(kn)->kernfs_rwsem)
 {
@@ -472,6 +476,16 @@ static void kernfs_drain(struct kernfs_node *kn)
 	lockdep_assert_held_write(&root->kernfs_rwsem);
 	WARN_ON_ONCE(kernfs_active(kn));
 
+	/*
+	 * Skip draining if already fully drained. This avoids draining and its
+	 * lockdep annotations for nodes which have never been activated
+	 * allowing embedding kernfs_remove() in create error paths without
+	 * worrying about draining.
+	 */
+	if (atomic_read(&kn->active) == KN_DEACTIVATED_BIAS &&
+	    kernfs_should_drain_open_files(kn))
+		return false;
+
 	up_write(&root->kernfs_rwsem);
 
 	if (kernfs_lockdep(kn)) {
@@ -480,7 +494,6 @@ static void kernfs_drain(struct kernfs_node *kn)
 			lock_contended(&kn->dep_map, _RET_IP_);
 	}
 
-	/* but everyone should wait for draining */
 	wait_event(root->deactivate_waitq,
 		   atomic_read(&kn->active) == KN_DEACTIVATED_BIAS);
 
@@ -493,6 +506,8 @@ static void kernfs_drain(struct kernfs_node *kn)
 		kernfs_drain_open_files(kn);
 
 	down_write(&root->kernfs_rwsem);
+
+	return true;
 }
 
 /**
@@ -1370,23 +1385,14 @@ static void __kernfs_remove(struct kernfs_node *kn)
 		pos = kernfs_leftmost_descendant(kn);
 
 		/*
-		 * kernfs_drain() drops kernfs_rwsem temporarily and @pos's
+		 * kernfs_drain() may drop kernfs_rwsem temporarily and @pos's
 		 * base ref could have been put by someone else by the time
 		 * the function returns.  Make sure it doesn't go away
 		 * underneath us.
 		 */
 		kernfs_get(pos);
 
-		/*
-		 * Drain iff @kn was activated.  This avoids draining and
-		 * its lockdep annotations for nodes which have never been
-		 * activated and allows embedding kernfs_remove() in create
-		 * error paths without worrying about draining.
-		 */
-		if (kn->flags & KERNFS_ACTIVATED)
-			kernfs_drain(pos);
-		else
-			WARN_ON_ONCE(atomic_read(&kn->active) != KN_DEACTIVATED_BIAS);
+		kernfs_drain(pos);
 
 		/*
 		 * kernfs_unlink_sibling() succeeds once per node.  Use it
-- 
2.37.2