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

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Date:   Fri, 5 Apr 2019 06:51:43 +0000
From:   "Vaittinen, Matti" <Matti.Vaittinen@...rohmeurope.com>
To:     "sboyd@...nel.org" <sboyd@...nel.org>,
        "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

Hello Stephen,

Thanks for taking care of this!

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?

Br,
	Matti Vaittinen