Re: [PATCH] kernfs: Separate kernfs_pr_cont_buf and rename_lock.

[Tejun Heo <tj@kernel.org>]

On Mon, May 16, 2022 at 11:28:59AM -0700, Hao Luo wrote:
> Previously the protection of kernfs_pr_cont_buf was piggy backed by
> rename_lock, which means that pr_cont() needs to be protected under
> rename_lock. This can cause potential circular lock dependencies.
> 
> If there is an OOM, we have the following call hierarchy:
> 
>  -> cpuset_print_current_mems_allowed()
>    -> pr_cont_cgroup_name()
>      -> pr_cont_kernfs_name()
> 
> pr_cont_kernfs_name() will grab rename_lock and call printk. So we have
> the following lock dependencies:
> 
>  kernfs_rename_lock -> console_sem
> 
> Sometimes, printk does a wakeup before releasing console_sem, which has
> the dependence chain:
> 
>  console_sem -> p->pi_lock -> rq->lock
> 
> Now, imagine one wants to read cgroup_name under rq->lock, for example,
> printing cgroup_name in a tracepoint in the scheduler code. They will
> be holding rq->lock and take rename_lock:
> 
>  rq->lock -> kernfs_rename_lock
> 
> Now they will deadlock.
> 
> A prevention to this circular lock dependency is to separate the
> protection of pr_cont_buf from rename_lock. In principle, rename_lock
> is to protect the integrity of cgroup name when copying to buf. Once
> pr_cont_buf has got its content, rename_lock can be dropped. So it's
> safe to drop rename_lock after kernfs_name_locked (and
> kernfs_path_from_node_locked) and rely on a dedicated pr_cont_lock
> to protect pr_cont_buf.
> 
> Signed-off-by: Hao Luo <haoluo@google.com>

Can you please add a comment explaining why the lock is separate? Other than
that:

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun