lkml.org 
[lkml]   [2013]   [May]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] clk: Fix race condition between clk_set_parent and clk_enable()
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@codeaurora.org>
>>
>> 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

--
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
hosted by The Linux Foundation


\
 
 \ /
  Last update: 2013-05-15 01:01    [W:0.049 / U:0.304 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site