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Date: Tue, 15 May 2012 12:51:06 -0700
From: Saravana Kannan <skannan@codeaurora.org>
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To: Sascha Hauer <s.hauer@pengutronix.de>
CC: Mike Turquette <mturquette@linaro.org>,
        Arnd Bergman <arnd.bergmann@linaro.org>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
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        Rob Herring <rob.herring@calxeda.com>,
        Russell King <linux@arm.linux.org.uk>,
        Jeremy Kerr <jeremy.kerr@canonical.com>,
        Thomas Gleixner <tglx@linutronix.de>, Paul Walmsley <paul@pwsan.com>,
        Shawn Guo <shawn.guo@freescale.com>, Jamie Iles <jamie@jamieiles.com>,
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        Magnus Damm <magnus.damm@gmail.com>,
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        Amit Kucheria <amit.kucheria@linaro.org>,
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        Grant Likely <grant.likely@secretlab.ca>
Subject: Re: [PATCH] clk: Fix race conditions between clk_set_parent() and
 clk_enable()
References: <1336798797-8724-1-git-send-email-skannan@codeaurora.org> <20120515194245.GO30400@pengutronix.de>
In-Reply-To: <20120515194245.GO30400@pengutronix.de>
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On 05/15/2012 12:42 PM, Sascha Hauer wrote:
> Hi Saravana,
>
> On Fri, May 11, 2012 at 09:59:56PM -0700, 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.
>
> Had a look at this and I can follow your reasoning and you patch seems
> to fix this. However, there is a problem,
>
>>
>>   	/* migrate prepare and enable */
>> @@ -1092,23 +1098,23 @@ static int __clk_set_parent(struct clk *clk, struct clk *parent)
>>
>>   	/* FIXME replace with clk_is_enabled(clk) someday */
>>   	spin_lock_irqsave(&enable_lock, flags);
>> +
>>   	if (clk->enable_count)
>>   		__clk_enable(parent);
>> -	spin_unlock_irqrestore(&enable_lock, flags);
>>
>>   	/* change clock input source */
>>   	ret = clk->ops->set_parent(clk->hw, i);
>
> You call ->set_parent while holding a spinlock. This won't work with i2c
> clocks.

I did account for that. I explained it in the commit text. Please let me 
know if any part of that is not clear or is not correct.

Thanks,
Saravana

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