Add two lockless checks before acquiring the lock: 1. READ_ONCE(cfile->kn) NULL check to skip torn-down files. 2. READ_ONCE(cfile->notified_at) check to skip when within the rate-limit window (~10ms). Both checks have safe error directions -- a stale read can only cause unnecessary lock acquisition, never a missed notification. Annotate all write sites with WRITE_ONCE() to pair with the lockless readers. The trade-off is that trailing timer_reduce() calls during bursts are skipped, so the deferred notification that delivers the final state may be lost. This is acceptable for the primary callers like __memcg_memory_event() where events keep arriving. Signed-off-by: Shakeel Butt Reported-by: Jakub Kicinski --- kernel/cgroup/cgroup.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-) diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index 33282c7d71e4..5473ebd0f6c1 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); } @@ -4686,20 +4686,27 @@ int cgroup_add_legacy_cftypes(struct cgroup_subsys *ss, struct cftype *cfts) */ void cgroup_file_notify(struct cgroup_file *cfile) { - unsigned long flags; + unsigned long flags, last, next; struct kernfs_node *kn = NULL; + if (!READ_ONCE(cfile->kn)) + return; + + last = READ_ONCE(cfile->notified_at); + if (time_before_eq(jiffies, last + CGROUP_FILE_NOTIFY_MIN_INTV)) + return; + spin_lock_irqsave(&cgroup_file_kn_lock, flags); if (cfile->kn) { - unsigned long last = cfile->notified_at; - unsigned long next = last + CGROUP_FILE_NOTIFY_MIN_INTV; + last = cfile->notified_at; + next = last + CGROUP_FILE_NOTIFY_MIN_INTV; - if (time_in_range(jiffies, last, next)) { + if (time_before_eq(jiffies, next)) { timer_reduce(&cfile->notify_timer, next); } else { kn = cfile->kn; kernfs_get(kn); - cfile->notified_at = jiffies; + WRITE_ONCE(cfile->notified_at, jiffies); } } spin_unlock_irqrestore(&cgroup_file_kn_lock, flags); -- 2.47.3