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

[Edward Adam Davis]

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.

Edward
BR