Re: [PATCH] iio: buffer: Silence lock nesting splat

[Vincent Whitchurch <vincent.whitchurch@axis.com>]

On Sat, Aug 20, 2022 at 01:08:28PM +0200, Lars-Peter Clausen wrote:
> There are two different approaches for this kind of nested locking. One 
> is to use mutex_lock_nested(). This works if there is a strict 
> hierarchy. The I2C framework for example has a function to determine the 
> position of a I2C mux in the hierarchy and uses that for locking. See 
> https://elixir.bootlin.com/linux/latest/source/drivers/i2c/i2c-core-base.c#L1151.
> 
> I'm not sure this directly translates to IIO since the 
> consumers/producers don't have to be a in strict hierarchy.  And if it 
> is a complex graph it can be difficult to figure out the right level for 
> mutex_lock_nested().
> 
> The other method is to mark each mutex as its own class. lockdep does 
> the lock checking based on the lock class and by default the same mutex 
> of different instances is considered the same class to keep the resource 
> requirements for the checker lower.
> 
> Regmap for example does this. See 
> https://elixir.bootlin.com/linux/latest/source/drivers/base/regmap/regmap.c#L795.
> 
> This could be a solution for IIO with the downside how the additional 
> work for the checker. But as long as there are only a few IIO devices 
> per system that should be OK. We could also only set the per device lock 
> class if in kernel consumers are enabled.

The second method certainly sounds like a better fix, since it also
still warns if one actually takes the same iio_dev mutex twice.  I'll
respin the patch.  Thanks.