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

[Saravana Kannan]

On 05/14/2013 03:10 PM, Tomasz Figa wrote:
> Hi,
>
> On Tuesday 14 of May 2013 11:54:17 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.  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.
>
> IMHO this behavior should be documented somewhere, with a note that the
> clock must not be prepared to keep it disabled during reparent operation
> and possibly also pointing to the CLK_SET_PARENT_GATE flag.

Reasonable request. I can update the documentation of clk_set_parent() 
to indicate that the clock might get turned on for the duration of the 
call and if they need a guarantee the GATE flag should be used.

> > 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.
>
> Well, on any platform where default clock settings are not completely
> correct this is likely to cause problems, because some device might get
> too high frequency for some period of time, which might crash it alone as
> well as the whole system.

I don't think this is really a problem with this patch. It's present 
even without this patch.

The patch doesn't switch to some other unspecified parent. It only 
switches between the new/old parent. Even without this patch, if a clock 
is prepared while you reparent it, clk_enable() could be called at 
anytime between the parent switch and the future clock API calls to set 
up the new parent correctly. I think that's just crappy driver code to 
switch to a new parent before setting it up correctly. There's 
absolutely no good reason to do it that way.

-Saravana

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