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@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. > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
