From mboxrd@z Thu Jan 1 00:00:00 1970 From: skannan@codeaurora.org (Saravana Kannan) Date: Tue, 14 May 2013 14:03:04 -0700 Subject: [PATCH] clk: Fix race condition between clk_set_parent and clk_enable() In-Reply-To: <20130514185417.10068.5717@quantum> References: <1367383328-25700-1-git-send-email-skannan@codeaurora.org> <20130514185417.10068.5717@quantum> Message-ID: <5192A688.3040007@codeaurora.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 05/14/2013 11:54 AM, 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: >> * 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 > > I've taken this patch into clk-next for testing. The code itself looks > fine. Thanks Mike. I'll send it out again with some typo/grammar corrections. > 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. I would think that would be a general issue with the clock APIs since disable doesn't guarantee a disable (since it's ref counted). Also, those clocks could be marked as CLK_SET_PARENT_GATE if it's a real issue. > 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. I can't think of any either, but as I mentioned, we have CLK_SET_PARENT_GATE for that. Thanks, Saravana -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation