Re: [PATCH] drm: Drop modeset locking from crtc init function

[Thomas Hellstrom <thellstrom@vmware.com>]

On 09/08/2014 09:03 AM, Daniel Vetter wrote:
> At driver init no one can access modeset objects and we're
> single-threaded. So locking is just cargo-culting here. Worse, with
> the new ww mutexes and ww mutex slowpath debugging the mutex_lock
> might actually fail, and we don't have the full-blown ww recovery
> dance.
>
> Which then leads to fireworks when we try to unlock the not-locked
> crtc lock.
>
> An audit of all the functions called from here shows that none of them
> contain locking checks, so there's also no reason to keep the locking
> around just for consistency of caller contexts. Besides that I have
> the rule (at least in i915) that such places where we take locks just
> to simplify locking checks and not for correctness always require a
> comment.

I'm not really opposed to any of the patches. It's clear that trylock
will work, and it's also clear that locking is not strictly needed, at
least not of a lock that has not been published yet.

However, I tend to go for the  "lock even if it's unnecessary" version
for a couple of reasons:

a) If that turns out to be impossible or very hard, then something is
probably wrong with the design.
b) It's good to think of locks where possible as "protecting data"
rather than serializing something. With that in mind, and if we in the
future were to have tools to automatically check that relevant locks are
held while accessing lock-protected stuff, we're in trouble.
c) Even if there aren't any functions now that check for relevant locks
held, there might be in the future.
d) People will probably spend time wondering why locking is done
elsewhere but not here.

So at least considering d) and b) I'd like to see documentation the
other way around:
If we avoid taking locks around data accesses that are supposed to be
protected by the lock for whatever reason, the reason should be documented.

Thanks,
Thomas