Re: [PATCH] clk: Fix race conditions between clk_set_parent() andclk_enable()

[Saravana Kannan]

On 05/15/2012 12:42 PM, Sascha Hauer wrote:
> Hi Saravana,
>
> On Fri, May 11, 2012 at 09:59:56PM -0700, 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.
>
> Had a look at this and I can follow your reasoning and you patch seems
> to fix this. However, there is a problem,
>
> >   	/* 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);
>
> You call ->set_parent while holding a spinlock. This won't work with i2c
> clocks.

I did account for that. I explained it in the commit text. Please let me 
know if any part of that is not clear or is not correct.

Thanks,
Saravana

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