Re: [PATCH 08/35] drm: Protect dev->filelist with its own mutex

From: Daniel Vetter <daniel@ffwll.ch>
To: Alex Deucher <alexdeucher@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>,
	Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
	DRI Development <dri-devel@lists.freedesktop.org>,
	Alex Deucher <alexander.deucher@amd.com>,
	Daniel Vetter <daniel.vetter@intel.com>
Subject: Re: [PATCH 08/35] drm: Protect dev->filelist with its own mutex
Date: Wed, 27 Apr 2016 09:08:41 +0200	[thread overview]
Message-ID: <20160427070841.GM2558@phenom.ffwll.local> (raw)
In-Reply-To: <20160427070609.GL2558@phenom.ffwll.local>

On Wed, Apr 27, 2016 at 09:06:09AM +0200, Daniel Vetter wrote:
> On Tue, Apr 26, 2016 at 05:45:44PM -0400, Alex Deucher wrote:
> > On Tue, Apr 26, 2016 at 4:52 PM, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > > On Tue, Apr 26, 2016 at 07:29:41PM +0200, Daniel Vetter wrote:
> > >> amdgpu gained dev->struct_mutex usage, and that's because it's walking
> > >> the dev->filelist list. Protect that list with it's own lock to take
> > >> one more step towards getting rid of struct_mutex usage in drivers
> > >> once and for all.
> > >>
> > >> While doing the conversion I noticed that 2 debugfs files in i915
> > >> completely lacked appropriate locking. Fix that up too.
> > >>
> > >> v2: don't forget to switch to drm_gem_object_unreference_unlocked.
> > >>
> > >> Cc: Alex Deucher <alexander.deucher@amd.com>
> > >> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > >
> > > Just wondering if this worth converting over. Opening/closing isn't
> > > going to be high contention, I hope, though we can certainly write
> > > stress cases for it! The goal for drivers to stop using the struct_mutex
> > > as their BKL, which doesn't preclude keeping the struct_mutex around for
> > > where it makes sense to have a single mutex rather than a multitude.
> > >
> > > I have some misgivings over this, but only because I think its overkill.
> > > Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
> > 
> > I agree with Chris' sentiments.
> 
> It's not to have more speed or less contention, but just to have fewer
> things to worry about when reviewing locking. Hence orthogonal locks for
> independent parts.
> 
> My goal is that in the end dev->struct_mutex is only used by some existing
> drivers for their internals, plus all the legacy core stuff. And never
> even used by modern drivers. New locks are pretty cheap, and not dragging
> in the entire legacy horror show has benefits.
> 
> When/once I tackle the one thing left (master locking) I might move the
> master handling under this lock too (since it's closely related to open
> files). Not sure yet.

Oh and the main reason I did it here: Much easier to review using git grep
struct_mutex than auditing codepaths. With this drivers have no reason to
ever grab struct_mutex (not even for debugfs or similar), since the last
remaining bit (master control) really shouldn't be a concern for drivers
ever.

That also makes reviewing new drivers simpler: Contains struct_mutex?
Reject it!

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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