[PATCH] cgroup: add lockless fast-path checks to cgroup_file_notify()

[Shakeel Butt]

Add two lockless checks before acquiring the lock:

1. READ_ONCE(cfile->kn) NULL check to skip torn-down files.
2. timer_pending() check to skip when a deferred notification
   timer is already armed.

Both checks have safe error directions -- a stale read can only
cause unnecessary lock acquisition, never a missed notification.

Annotate cfile->kn write sites with WRITE_ONCE() to pair with the
lockless reader.

Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Reported-by: Jakub Kicinski <kuba@xxxxxxxxxx>
---
 kernel/cgroup/cgroup.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 2b298a2cf206..6e816d27ee25 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1749,7 +1749,7 @@ static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
 		struct cgroup_file *cfile = (void *)css + cft->file_offset;
 
 		spin_lock_irq(&cgroup_file_kn_lock);
-		cfile->kn = NULL;
+		WRITE_ONCE(cfile->kn, NULL);
 		spin_unlock_irq(&cgroup_file_kn_lock);
 
 		timer_delete_sync(&cfile->notify_timer);
@@ -4430,7 +4430,7 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
 		timer_setup(&cfile->notify_timer, cgroup_file_notify_timer, 0);
 
 		spin_lock_irq(&cgroup_file_kn_lock);
-		cfile->kn = kn;
+		WRITE_ONCE(cfile->kn, kn);
 		spin_unlock_irq(&cgroup_file_kn_lock);
 	}
 
@@ -4689,6 +4689,12 @@ void cgroup_file_notify(struct cgroup_file *cfile)
 	unsigned long flags;
 	struct kernfs_node *kn = NULL;
 
+	if (!READ_ONCE(cfile->kn))
+		return;
+
+	if (timer_pending(&cfile->notify_timer))
+		return;
+
 	spin_lock_irqsave(&cgroup_file_kn_lock, flags);
 	if (cfile->kn) {
 		unsigned long last = cfile->notified_at;
-- 
2.47.3