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SubjectRe: clk: Per controller locks (prepare & enable)
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On 06/30/2016 06:22 PM, Javier Martinez Canillas wrote:
>> Question:
>> What do you think about it? I know that talk is cheap and code looks
>> better but before starting the work I would like to hear some
>> comments/opinions/ideas.
>>
>
> The problem is that the enable and prepare operations are propagated to
> the parents, so what the locks want to protecting is really a sub-tree
> of the clock tree. They currently protect the whole clock hierarchy to
> make sure that the changes in the clock tree are atomically.

Although there is a hierarchy between clocks from different controllers
but still these are all clocks controllers coming from one hardware
device (like SoC). At least on Exynos, I think there is no real
inter-device dependencies. The deadlock you mentioned (and which I want
to fix) is between:
1. clock in PMIC (the one needed by s3c_rtc_probe()),
2. clock for I2C in SoC (the one needed by regmap_write()),
3. and regmap lock:

What I want to say is that the relationship between clocks even when
crossing clock controller boundaries is still self-contained. It is
simple parent-child relationship so acquiring both
clock-controller-level locks is safe.

Current dead lock looks like, simplifying your code:
A: B:
lock(regmap)
lock(prepare)
lock(prepare) - wait
lock(regmap) - wait


When split locks per clock controller this would be:
A: B:
lock(regmap)
lock(s2mps11)
lock(i2c/exynos)
lock(regmap) - wait
do the transfer
unlock(i2c/exynos)
unlock(regmap)
lock(regmap) - acquired
lock(i2c/exynos)
do the transfer
unlock(i2c/exynos)
unlock(regmap)
unlock(s2mps11)

I still have to figure out how to properly protect the entire tree
hierarchy. Maybe the global prepare_lock should be used only for that -
for traversing the hierarchy.

>
> So a clock per controller won't suffice since you can have a parent for
> a clock provided by a different controller and that won't be protected.
>
> Another option is to have a prepare and enable locks per clock. Since
> the operations are always propagated in the same direction, I think is
> safe to do it.
>
> But even in the case of a more fine-grained locking, I believe the
> mentioned deadlocks can still happen. For example in 10ff4c5239a1 the
> issue was that the s2mps11 PMIC has both regulators and clocks that are
> controlled via I2C so the regulator and clocks drivers shares the same
> I2C regmap.
>
> What happened was something like this:
>
> CPU0 CPU1
> ---- ----
> regulator_set_voltage() s3c_rtc_probe()
> regmap_write() clk_prepare()
> /* regmap->lock was aquired */ /* prepare_lock was aquired */
> regmap_i2c_write() s2mps11_clk_prepare()
> i2c_transfer() regmap_write()
> exynos5_i2c_xfer() /* deadlock due regmap->lock */
> clk_prepare_enable()
> clk_prepare_lock()
> /* prepare_lock was aquired */
>
> So if for example a per clock lock is used, the deadlock can still happen
> if both the I2C clock and S3C RTC source clock share the same parent in a
> part of the hierarchy. But as you said this is less likely in practice so
> probably is not an issue.

I think these clocks do not share the parent.

Best regards,
Krzysztof

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