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

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Add lockless checks before acquiring cgroup_file_kn_lock:

1. READ_ONCE(cfile->kn) NULL check to skip torn-down files.
2. READ_ONCE(cfile->notified_at) rate-limit check to skip when
   within the notification interval.  If within the interval, arm
   the deferred timer via timer_reduce() and confirm it is pending
   before returning -- if the timer fired in between, fall through
   to the lock path so the notification is not lost.

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

The critical section is simplified to just taking a kernfs_get()
reference and updating notified_at.

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

Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Shakeel Butt
2026-03-10 18:01:00 -07:00
committed by Tejun Heo
parent 05070cd654
commit 4616120fca
1 changed files with 17 additions and 13 deletions
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30
kernel/cgroup/cgroup.c
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@@ -1694,7 +1694,7 @@ 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);
@@ -4375,7 +4375,7 @@ 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);
}
@@ -4631,21 +4631,25 @@ 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);
next = last + CGROUP_FILE_NOTIFY_MIN_INTV;
if (time_in_range(jiffies, last, next)) {
timer_reduce(&cfile->notify_timer, next);
if (timer_pending(&cfile->notify_timer))
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;
if (time_in_range(jiffies, last, next)) {
timer_reduce(&cfile->notify_timer, next);
} else {
kn = cfile->kn;
kernfs_get(kn);
cfile->notified_at = jiffies;
}
kn = cfile->kn;
kernfs_get(kn);
WRITE_ONCE(cfile->notified_at, jiffies);
}
spin_unlock_irqrestore(&cgroup_file_kn_lock, flags);