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Subject[PATCH 5/5] cgroup: fix a race between cgroup_mount() and cgroup_kill_sb()
We've converted cgroup to kernfs so cgroup won't be intertwined with
vfs objects and locking, but there are dark areas.

Run two instances of this script concurrently:

for ((; ;))
{
mount -t cgroup -o cpuacct xxx /cgroup
umount /cgroup
}

After a while, I saw two mount processes were stuck at retrying, because
they were waiting for a subsystem to become free, but the root associated
with this subsystem never got freed.

This can happen, if thread A is in the process of killing superblock but
hasn't called percpu_ref_kill(), and at this time thread B is mounting
the same cgroup root and finds the root in the root list and performs
percpu_ref_try_get().

To fix this, we increase the refcnt of the superblock instead of increasing
the percpu refcnt of cgroup root.

Signed-off-by: Li Zefan <lizefan@huawei.com>
---

A better fix is welcome!

---
kernel/cgroup.c | 24 ++++++++++++++----------
1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index bd37e8d..94e1814 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1654,7 +1654,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
struct dentry *dentry;
int ret;
int i;
- bool new_sb;
+ bool sb_pinned = false;

/*
* The first time anyone tries to mount a cgroup, enable the list
@@ -1735,19 +1735,21 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
}

/*
- * A root's lifetime is governed by its root cgroup.
- * tryget_live failure indicate that the root is being
- * destroyed. Wait for destruction to complete so that the
- * subsystems are free. We can use wait_queue for the wait
- * but this path is super cold. Let's just sleep for a bit
- * and retry.
+ * This may fail for two reasons:
+ * - A concurrent mount is in process. We wait for that mount
+ to complete.
+ * - The superblock is being destroyed. We wait for the
+ * desctruction to complete so that the subsystems are free.
+ * We can use wait_queue for the wait but this path is super
+ * cold. Let's just sleep for a bit and retry.
*/
- if (!percpu_ref_tryget_live(&root->cgrp.self.refcnt)) {
+ if (!kernfs_pin_sb(root->kf_root, NULL)) {
mutex_unlock(&cgroup_mutex);
msleep(10);
ret = restart_syscall();
goto out_free;
}
+ sb_pinned = true;

ret = 0;
goto out_unlock;
@@ -1784,8 +1786,10 @@ out_free:
if (ret)
return ERR_PTR(ret);

- dentry = kernfs_mount(fs_type, flags, root->kf_root, &new_sb);
- if (IS_ERR(dentry) || !new_sb)
+ dentry = kernfs_mount(fs_type, flags, root->kf_root, NULL);
+ if (sb_pinned)
+ kernfs_drop_sb(root->kf_root, NULL);
+ if (!sb_pinned && IS_ERR(dentry))
cgroup_put(&root->cgrp);
return dentry;
}
--
1.8.0.2


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