linux-kernel - Re: [PATCH v3 1/7] clkdev: Hold clocks

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Message-ID: <155449664462.20095.10904772826291270300@swboyd.mtv.corp.google.com>
Date:   Fri, 05 Apr 2019 13:37:24 -0700
From:   Stephen Boyd <sboyd@...nel.org>
To:     "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>,
        "mturquette@...libre.com" <mturquette@...libre.com>
Cc:     "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "wens@...e.org" <wens@...e.org>,
        "miquel.raynal@...tlin.com" <miquel.raynal@...tlin.com>,
        "jhugo@...eaurora.org" <jhugo@...eaurora.org>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        "jbrunet@...libre.com" <jbrunet@...libre.com>
Subject: Re: [PATCH v3 1/7] clkdev: Hold clocks_mutex while iterating clocks list

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.