[PATCH] clk: Fix race conditions between clk_set_parent() and clk_enable()

[Saravana Kannan]

On 05/11/2012 09:59 PM, Saravana Kannan wrote:
> Without this patch, the following race conditions are possible.
>
> Race condition 1:
> * clk-A has two parents - clk-X and clk-Y.
> * All three are disabled and clk-X is current parent.
> * Thread A: clk_set_parent(clk-A, clk-Y).
> * Thread A:<snip execution flow>
> * Thread A: Grabs enable lock.
> * Thread A: Sees enable count of clk-A is 0, so doesn't enable clk-Y.
> * Thread A: Releases enable lock.
> * Thread B: Calls clk_enable(clk-A), which in turn enables clk-X.
> * Thread A: Switches clk-A's parent to clk-Y in hardware.
>
> clk-A is now enabled in software, but not clocking in hardware.
>
> Race condition 2:
> * clk-A has two parents - clk-X and clk-Y.
> * All three are disabled and clk-X is current parent.
> * Thread A: clk_set_parent(clk-A, clk-Y).
> * Thread A:<snip execution flow>
> * Thread A: Switches parent in hardware to clk-Y.
> * Thread A: Grabs enable lock.
> * Thread A: Sees enable count of clk-A is 0, so doesn't disable clk-X.
> * Thread A: Releases enable lock.
> * Thread B: Calls clk_enable(clk-A)
> * Thread B: Software state still says parent is clk-X.
> * Thread B: So, enables clk-X and then itself.
> * Thread A: Updates parent in software state to clk-Y.
>
> clk-A is now enabled in software, but not clocking in hardware. clk-X will
> never be disabled since it's enable count is 1 when no one needs it. clk-Y
> will throw a warning when clk-A is disabled again (assuming clk-A being
> disabled in hardware hasn't wedged the system).
>
> To fix these race conditions, hold the enable lock while switching the clock
> parent in hardware. But this would force the set_parent() ops to be atomic,
> which might not be possible if the clock hardware is external to the SoC.
>
> Since clocks with CLK_SET_PARENT_GATE flag only allow clk_set_parent() on
> unprepared clocks and calling clk_enable() on an unprepared clock would be
> violating the clock API usage model, allow set_parent() ops to be sleepable
> for clocks which have the CLK_SET_PARENT_GATE flag. Putting it another way,
> if a clock's parent can't be switched without sleeping, then by definition
> the parent can't be switched while it's prepared (CLK_SET_PARENT_GATE).
>
> Signed-off-by: Saravana Kannan<skannan at codeaurora.org>
> Cc: Mike Turquette<mturquette at linaro.org>
> Cc: Andrew Lunn<andrew at lunn.ch>
> Cc: Rob Herring<rob.herring at calxeda.com>
> Cc: Russell King<linux at arm.linux.org.uk>
> Cc: Jeremy Kerr<jeremy.kerr at canonical.com>
> Cc: Thomas Gleixner<tglx at linutronix.de>
> Cc: Arnd Bergman<arnd.bergmann at linaro.org>
> Cc: Paul Walmsley<paul at pwsan.com>
> Cc: Shawn Guo<shawn.guo at freescale.com>
> Cc: Sascha Hauer<s.hauer at pengutronix.de>
> Cc: Jamie Iles<jamie at jamieiles.com>
> Cc: Richard Zhao<richard.zhao at linaro.org>
> Cc: Saravana Kannan<skannan at codeaurora.org>
> Cc: Magnus Damm<magnus.damm at gmail.com>
> Cc: Mark Brown<broonie at opensource.wolfsonmicro.com>
> Cc: Linus Walleij<linus.walleij at stericsson.com>
> Cc: Stephen Boyd<sboyd at codeaurora.org>
> Cc: Amit Kucheria<amit.kucheria at linaro.org>
> Cc: Deepak Saxena<dsaxena at linaro.org>
> Cc: Grant Likely<grant.likely at secretlab.ca>
> ---
> Additional comments that I'm not sure are fit for the commit text:
>
> Reason for repeating the call to set_parent() ops and updating clk->parent
> for the CLK_SET_PARENT_GATE case:
> * It looks weird to wrap the migration code and the lock/unlock in separate
>    if's.
> * Once we add proper error checking for the return value of set_parent()
>    ops, the code will look even more convoluted if we try to share the code
>    for CLK_SET_PARENT_GATE case and non-CLK_SET_PARENT_GATE case.
>
> I realize that clk->parent = parent is repeated in __clk_reparent(). But I
> left that as is for now in case anyone is using that in one of for-next
> branches. If no one is using it, we can remove it.
>
> For a similar reason, clocks that need to do reparenting during
> clk_set_rate() and don't have CLK_SET_RATE_GATE set can't do it correctly
> with the current clk-provider APIs provided by the common clock framework.
>
> __clk_set_parent() should really be split into two APIs for this to work:
> __clk_pre_reparent() - enable lock grabbed here.
> __clk_post_reparent() - enable lock released here.
>
>   drivers/clk/clk.c |   16 +++++++++++-----
>   1 files changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> index e5d5dc1..09b9112 100644
> --- a/drivers/clk/clk.c
> +++ b/drivers/clk/clk.c
> @@ -1059,7 +1059,7 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent)
>   {
>   	struct clk *old_parent;
>   	unsigned long flags;
> -	int ret = -EINVAL;
> +	int ret;
>   	u8 i;
>
>   	old_parent = clk->parent;
> @@ -1083,7 +1083,13 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent)
>   	if (i == clk->num_parents) {
>   		pr_debug("%s: clock %s is not a possible parent of clock %s\n",
>   				__func__, parent->name, clk->name);
> -		goto out;
> +		return -EINVAL;
> +	}
> +
> +	if (clk->flags&  CLK_SET_PARENT_GATE) {
> +		ret = clk->ops->set_parent(clk->hw, i);
> +		clk->parent = parent;
> +		return ret;
>   	}
>
>   	/* migrate prepare and enable */
> @@ -1092,23 +1098,23 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent)
>
>   	/* FIXME replace with clk_is_enabled(clk) someday */
>   	spin_lock_irqsave(&enable_lock, flags);
> +
>   	if (clk->enable_count)
>   		__clk_enable(parent);
> -	spin_unlock_irqrestore(&enable_lock, flags);
>
>   	/* change clock input source */
>   	ret = clk->ops->set_parent(clk->hw, i);
> +	clk->parent = parent;
>
>   	/* clean up old prepare and enable */
> -	spin_lock_irqsave(&enable_lock, flags);
>   	if (clk->enable_count)
>   		__clk_disable(old_parent);
> +
>   	spin_unlock_irqrestore(&enable_lock, flags);
>
>   	if (clk->prepare_count)
>   		__clk_unprepare(old_parent);
>
> -out:
>   	return ret;
>   }
>

(*nudge*) Thoughts anyone?

-Saravana

-- 
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