Re: [PATCH v2 07/22] drm/msm: Do rpm get sooner in the submit path

[Viresh Kumar]

On 20-10-20, 12:56, Daniel Vetter wrote:
> Yeah that's bad practice. Generally you shouldn't need to hold locks
> in setup/teardown code, since there's no other thread which can
> possible hold a reference to anything your touching anymore. Ofc
> excluding quickly grabbing/dropping a lock to insert/remove objects
> into lists and stuff.
> 
> The other reason is that especially with anything related to sysfs or
> debugfs, the locking dependencies you're pulling in are enormous: vfs
> locks pull in mm locks (due to mmap) and at that point there's pretty
> much nothing left you're allowed to hold while acquiring such a lock.
> For simple drivers this is no issue, but for fancy drivers (like gpu
> drivers) which need to interact with core mm) this means your
> subsystem is a major pain to use.
> 
> Usually the correct fix is to only hold your subsystem locks in
> setup/teardown when absolutely required, and fix any data
> inconsistency issues by reordering your setup/teardown code: When you
> register as the last step and unregister as the first step, there's no
> need for any additional locking. And hence no need to call debugfs
> functions while holding your subsystem locks.
> 
> The catch phrase I use for this is "don't solve object lifetime issues
> with locking". Instead use refcounting and careful ordering in
> setup/teardown code.

This is exactly what I have done in the OPP core, the locks were taken
only when really necessary, though as we have seen now I have missed
that at a single place and that should be fixed as well. Will do that,
thanks.

-- 
viresh