On Mon, 2019-04-08 at 23:21 +0100, Russell King - ARM Linux admin wrote: > On Mon, Apr 08, 2019 at 10:00:02AM -0700, Stephen Boyd wrote: > > Quoting Matti Vaittinen (2019-04-08 03:49:41) > > > On Fri, Apr 05, 2019 at 01:37:24PM -0700, Stephen Boyd wrote: > > > > 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. > > > > > > If the only (intended) user for clk_find is > > > devm_clk_release_clkdev, > > > then we might want to write it in devm_clk_release_clkdev - just > > > to > > > avoid similar errors (as I did with devm) in the future? I might > > > even > > > consider renaming __clk_find to clk_find or to clk_find_unsafe - > > > but > > > that's all just nitpicking :) Go with what you like to maintain > > > :D > > > > > > > Sure. I was thinking along the same lines after you asked. > > This is rubbish. The reason clk_find() doesn't take the lock is that > you _need_ to hold the lock while you dereference the clk_lookup > data.
I think we all agreed on this already. Stephen pointed out that the current user(s) of clk_find() do _not_ dereference the pointer. > The lock isn't protecting just the lookup, it protects what you do > with > the result of the lookup as well. And we agreed on this too. Br, Matti Vaittinen