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

[Turquette, Mike]

On Tue, May 22, 2012 at 6:58 AM, Peter De Schrijver
<pdeschrijver@xxxxxxxxxx> wrote:
> On Tue, May 15, 2012 at 08:20:44PM +0200, Saravana Kannan wrote:
>> On 05/11/2012 09:59 PM, 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.
>> >
>
> This looks correct to me. Is there any usecase where enabling/disabling a
> clock would require sleeping but changing the parent would not?
>

clk_enable & clk_disable must never sleep.  clk_prepare and
clk_unprepare may sleep.

This patch would require the WM831x clock driver to set
CLK_SET_PARENT_GATE for the clkout mux.  The latest version of this
driver sent out by Mark does not do this and I imagine that is because
the hardware has no requirement for the mux clock to gate while
switching parents.  Holding the spinlock across the .set_parent
callback breaks this driver since .set_parent results in an i2c
transaction.

Mark, do you have an opinion on the requirements that this patch
imposes on your driver?

Regards,
Mike

> Cheers,
>
> Peter.
>
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