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

[Peter De Schrijver]

On Tue, May 22, 2012 at 08:06:45PM +0200, Turquette, Mike wrote:
> On Tue, May 22, 2012 at 6:58 AM, Peter De Schrijver
> <pdeschrijver at nvidia.com> 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.
> 

In that case the clock is actually enabled in clk_prepare and disabled in
clk_unprepare I guess (and clk_enable/clk_disable are dummy functions)?

What I'm trying to say is that I don't think there are clocks which can be
enabled/disabled using non blocking operations, but where a parent change
would require a blocking operation.

Cheers,

Peter.