Quoting Vaittinen, Matti (2019-04-04 23:51:43) > On Thu, 2019-04-04 at 14:53 -0700, Stephen Boyd wrote: > > We recently introduced a change to support devm clk lookups. That > > change > > introduced a code-path that used clk_find() without holding the > > 'clocks_mutex'. Unfortunately, clk_find() iterates over the 'clocks' > > list and so we need to prevent the list from being modified while > > iterating over it by holding the mutex. Similarly, we don't need to > > hold > > the 'clocks_mutex' besides when we're dereferencing the clk_lookup > > pointer > > /// Snip > > > -out: > > +static struct clk_lookup *clk_find(const char *dev_id, const char > > *con_id) > > +{ > > + struct clk_lookup *cl; > > + > > + mutex_lock(&clocks_mutex); > > + cl = __clk_find(dev_id, con_id); > > mutex_unlock(&clocks_mutex); > > > > - return cl ? clk : ERR_PTR(-ENOENT); > > + return cl; > > +} > > I am not an expert on this but reading commit message abowe and seeing > the code for clk_find() looks a bit scary. If I understand it > correctly, the clocks_mutex should be held when dereferencing the > clk_lookup returned by clk_find. The clk_find implementation drops the > lock before returning - which makes me think I miss something here. How > can the caller ever safely dereference returned clk_lookup pointer? > Just reading abowe makes me think that lock should be taken by whoever > is calling the clk_find, and dropped only after caller has used the > found clk_lookup for whatever caller intends to use it. Maybe I am > missing something? >
The only user after this patch (devm) is doing a pointer comparison so it looks OK. But yes, in general there shouldn't be users of clk_find() that dereference the pointer because there isn't any protection besides the mutex.