Messages in this thread | | | Date | Mon, 8 Apr 2019 13:49:41 +0300 | From | Matti Vaittinen <> | Subject | Re: [PATCH v3 1/7] clkdev: Hold clocks_mutex while iterating clocks list |
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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
Best regards Matti Vaittinen
-- Matti Vaittinen, Linux device drivers ROHM Semiconductors, Finland SWDC Kiviharjunlenkki 1E 90220 OULU FINLAND
~~~ "I don't think so," said Rene Descartes. Just then he vanished ~~~
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