From: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
To: Stephen Boyd <sboyd@kernel.org>
Cc: "mturquette@baylibre.com" <mturquette@baylibre.com>,
"linux@armlinux.org.uk" <linux@armlinux.org.uk>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"wens@csie.org" <wens@csie.org>,
"miquel.raynal@bootlin.com" <miquel.raynal@bootlin.com>,
"jhugo@codeaurora.org" <jhugo@codeaurora.org>,
"linux-clk@vger.kernel.org" <linux-clk@vger.kernel.org>,
"jbrunet@baylibre.com" <jbrunet@baylibre.com>
Subject: Re: [PATCH v3 1/7] clkdev: Hold clocks_mutex while iterating clocks list
Date: Mon, 8 Apr 2019 13:49:41 +0300 [thread overview]
Message-ID: <20190408104941.GB17594@localhost.localdomain> (raw)
In-Reply-To: <155449664462.20095.10904772826291270300@swboyd.mtv.corp.google.com>
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 ~~~
next prev parent reply other threads:[~2019-04-08 10:49 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-04 21:53 [PATCH v3 0/7] Rewrite clk parent handling Stephen Boyd
2019-04-04 21:53 ` [PATCH v3 1/7] clkdev: Hold clocks_mutex while iterating clocks list Stephen Boyd
2019-04-05 6:51 ` Vaittinen, Matti
2019-04-05 20:37 ` Stephen Boyd
2019-04-08 10:49 ` Matti Vaittinen [this message]
2019-04-08 17:00 ` Stephen Boyd
2019-04-08 22:21 ` Russell King - ARM Linux admin
2019-04-09 5:31 ` Vaittinen, Matti
2019-04-09 8:57 ` Russell King - ARM Linux admin
2019-04-10 16:45 ` Stephen Boyd
2019-04-04 21:53 ` [PATCH v3 2/7] clk: Prepare for clk registration API that uses DT nodes Stephen Boyd
2019-04-04 21:53 ` [PATCH v3 3/7] clk: Add of_clk_hw_register() API for early clk drivers Stephen Boyd
2019-04-08 21:46 ` Jeffrey Hugo
2019-04-10 16:49 ` Stephen Boyd
2019-04-10 16:53 ` Stephen Boyd
2019-04-10 19:39 ` Jeffrey Hugo
2019-04-10 21:45 ` Stephen Boyd
2019-04-10 22:18 ` Jeffrey Hugo
2019-04-11 17:32 ` Jeffrey Hugo
2019-04-04 21:53 ` [PATCH v3 4/7] clk: Allow parents to be specified without string names Stephen Boyd
2019-04-04 21:53 ` [PATCH v3 5/7] clk: Look for parents with clkdev based clk_lookups Stephen Boyd
2019-04-04 21:53 ` [PATCH v3 6/7] clk: Allow parents to be specified via clkspec index Stephen Boyd
2019-04-04 21:53 ` [PATCH v3 7/7] clk: fixed-factor: Let clk framework find parent Stephen Boyd
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