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

[Saravana Kannan]

On 05/14/2013 11:54 AM, Mike Turquette wrote:
> Quoting Saravana Kannan (2013-04-30 21:42:08)
> > 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: Updates clk-A SW parent to clk-Y
> > * Thread A: Releases enable lock.
> > * Thread B: clk_enable(clk-A).
> > * Thread B: clk_enable() enables clk-Y, then enabled clk-A and returns.
> >
> > clk-A is now enabled in software, but not clocking in hardware since the
> > hardware parent is still clk-X.
> >
> > The only way to avoid race conditions between clk_set_parent() and
> > clk_enable/disable() is to ensure that clk_enable/disable() calls don't
> > require changes to hardware enable state between changes to software clock
> > topology and hardware clock topology.
> >
> > There are options to achieve the above:
> > 1. Grab the enable lock before changing software/hardware topology and
> >     release it afterwards.
> > 2. Keep the clock enabled for the duration of software/hardware topology
> >     change so that any additional enable/disable calls don't try to change
> >     the hardware state. Once the topology change is complete, the clock can
> >     be put back in its original enable state.
> >
> > Option (1) is not an acceptable solution since the set_parent() ops might
> > need to sleep.
> >
> > Therefore, this patch implements option (2).
> >
> > This patch doesn't violate any API semantics. clk_disable() doesn't
> > guarantee that the clock is actually disabled. So, no clients of a clock
> > can assume that a clock is disabled after their last call to clk_disable().
> > So, enabling the clock during a parent change is not a violation of any API
> > semantics.
> >
> > This also has the nice side effect of simplifying the error handling code.
> >
> > Signed-off-by: Saravana Kannan <skannan@xxxxxxxxxxxxxx>
>
> I've taken this patch into clk-next for testing.  The code itself looks
> fine.

Thanks Mike. I'll send it out again with some typo/grammar corrections.

> The only thing that remains to be seen is if any platforms have a
> problem with disabled clocks getting turned on during a reparent
> operation.

I would think that would be a general issue with the clock APIs since 
disable doesn't guarantee a disable (since it's ref counted).

Also, those clocks could be marked as CLK_SET_PARENT_GATE if it's a real 
issue.

> On platforms that I have worked on this is OK, but I suppose there could
> be some platform out there where a clock is prepared and disabled, and
> briefly enabling the clock during the reparent operation somehow puts
> the hardware in a bad state.

I can't think of any either, but as I mentioned, we have 
CLK_SET_PARENT_GATE for that.

Thanks,
Saravana

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