On Tue, Sep 3, 2019 at 4:53 AM Michal Koutný [off-list ref] wrote:

On Mon, Sep 02, 2019 at 04:02:57PM -0700, Suren Baghdasaryan [off-list ref] wrote:

quoted

quoted

+static inline void cpu_uclamp_print(struct seq_file *sf,
+                                   enum uclamp_id clamp_id)
[...]
+       rcu_read_lock();
+       tg = css_tg(seq_css(sf));
+       util_clamp = tg->uclamp_req[clamp_id].value;
+       rcu_read_unlock();
+
+       if (util_clamp == SCHED_CAPACITY_SCALE) {
+               seq_puts(sf, "max\n");
+               return;
+       }
+
+       percent = tg->uclamp_pct[clamp_id];

You are taking RCU lock when accessing tg->uclamp_req but not when
accessing tg->uclamp_pct.

Good point.

quoted

Is that intentional? Can tg be destroyed under you?

Actually, the rcu_read{,un}lock should be unnecessary in the context of
the kernfs file op handler -- the tg/css won't go away as long as its
kernfs file is being worked with.

Also, add to that the fact that there is no rcu_dereference() call to
access any of the pointers in the reader or any of its callers. And, I
don't see any "wait for completion" type of pattern here so that
rcu_read_{lock, unlock}() pair does seem useless.

thanks,

 - Joel