Subject: [PATCH v2] clk: Fix race condition between clk_set_parent and clk_enable()

From: [email-masked] (Saravana Kannan)
Date: Thu, 16 May 2013 14:31:23 -0700

On 05/16/2013 01:44 PM, Mike Turquette wrote:
> Quoting Saravana Kannan (2013-05-15 21:07:24)
>> Without this patch, the following race condition is possible.
>> * 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.
>>
>> The options to achieve the above are:
>> 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 <[email-masked]>
>
> Updated to this version in clk-next.
>

Thanks Mike. I forgot to add the Ack by Ulf. Would be nice if you can 
put that in.

Btw, I did send this email to the list. But looks like this mail is 
wedged in the series of tubes in the arm mailing list.

-Saravana

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