commit 205a872bd6f9a9a09ef035ef1e90185a8245cc58 ("cgroup: fix lockdep
warning for event_control") sovled a deadlock by introducing a new
bug.
We can't access @event without event_list_lock, otherwise we'll race
with cgroup_event_wake() called when closing the eventfd, and then
both threads will try to free the same @event.
CPU0 CPU1
--------------------------- -----------------------------
cgroup_rmdir() close(eventfd)
list_for_each_entry() cgroup_event_wake()
list_del(event)
list_del(event)
cgroup_event_remove(event)
cgroup_event_remove(event)
To fix this, use the new eventfd_signal_hangup() to notify userspace
cgroup is removed.
Signed-off-by: Li Zefan <lizefan@xxxxxxxxxx>
---
kernel/cgroup.c | 31 +++++++++++++++++++------------
1 file changed, 19 insertions(+), 12 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 3d21adf..a3d361b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4302,9 +4302,9 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
struct dentry *d = cgrp->dentry;
struct cgroup *parent = cgrp->parent;
DEFINE_WAIT(wait);
- struct cgroup_event *event, *tmp;
+ struct eventfd_ctx *ctx;
+ struct cgroup_event *event;
struct cgroup_subsys *ss;
- LIST_HEAD(tmp_list);
lockdep_assert_held(&d->d_inode->i_mutex);
lockdep_assert_held(&cgroup_mutex);
@@ -4359,20 +4359,27 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
/*
* Unregister events and notify userspace.
* Notify userspace about cgroup removing only after rmdir of cgroup
- * directory to avoid race between userspace and kernelspace. Use
- * a temporary list to avoid a deadlock with cgroup_event_wake(). Since
+ * directory to avoid race between userspace and kernelspace. Since
* cgroup_event_wake() is called with the wait queue head locked,
- * remove_wait_queue() cannot be called while holding event_list_lock.
+ * eventfd_signal() cannot be called while holding event_list_lock.
*/
spin_lock(&cgrp->event_list_lock);
- list_splice_init(&cgrp->event_list, &tmp_list);
- spin_unlock(&cgrp->event_list_lock);
- list_for_each_entry_safe(event, tmp, &tmp_list, list) {
- list_del_init(&event->list);
- remove_wait_queue(event->wqh, &event->wait);
- eventfd_signal(event->eventfd, 1);
- schedule_work(&event->remove);
+ while (true) {
+ if (list_empty(&cgrp->event_list))
+ break;
+
+ event = list_first_entry(&cgrp->event_list,
+ struct cgroup_event, list);
+ ctx = eventfd_ctx_get(event->eventfd);
+ spin_unlock(&cgrp->event_list_lock);
+
+ eventfd_signal(ctx, 1);
+ eventfd_signal_hangup(ctx);
+ eventfd_ctx_put(ctx);
+
+ spin_lock(&cgrp->event_list_lock);
}
+ spin_unlock(&cgrp->event_list_lock);
return 0;
}
--
1.8.0.2
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