Re: [PATCH v2] sched/psi: fix race between file release and pressure write

[Chen Ridong]

On 2026/4/11 12:25, Edward Adam Davis wrote:
> On Fri, 10 Apr 2026 09:14:05 -1000, Tejun Heo wrote:
>>>  static ssize_t pressure_write(struct kernfs_open_file *of, char *buf,
>>>  			      size_t nbytes, enum psi_res res)
>>>  {
>>> -	struct cgroup_file_ctx *ctx = of->priv;
>>> +	struct cgroup_file_ctx *ctx;
>>>  	struct psi_trigger *new;
>>>  	struct cgroup *cgrp;
>>>  	struct psi_group *psi;
>>> +	ssize_t ret = 0;
>>>
>>>  	cgrp = cgroup_kn_lock_live(of->kn, false);
>>>  	if (!cgrp)
>>>  		return -ENODEV;
>>>
>>> +	ctx = of->priv;
>>> +	if (!ctx) {
>>
>> This test likely isn't necessary but that's pre-existing.
> Where?
> Are you referring to the check for of->released within:
> 'kernfs_fop_write_iter()->kernfs_get_active_of()'? This check is not
> performed under the protection of the cgroup_mutex; consequently, it
> is susceptible to race conditions, rendering the value unreliable, as
> it could be updated at any moment.
>>
>>> +		ret = -ENODEV;
>>> +		goto out_unlock;
>>> +	}
>>> +
>>>  	cgroup_get(cgrp);
>>
>> We don't need get/put if we don't drop the mutex, right?
> I believe that is indeed the case; the cgroup_get() call here is intended
> to facilitate subsequent operations, such as executing an smp store.
> 

Sorry, I don’t quite understand why get/put is needed. Could you elaborate a bit
more?

-- 
Best regards,
Ridong