[PATCH 00/18] dev->struct_mutex crusade

Daniel Vetter daniel at ffwll.ch
Mon Aug 10 05:00:19 PDT 2015

On Mon, Aug 10, 2015 at 01:35:58PM +0200, Thierry Reding wrote:
> On Thu, Jul 09, 2015 at 11:32:32PM +0200, Daniel Vetter wrote:
> > Hi all,
> > 
> > I wanted to take another look at struct_mutex usage in modern (gem) drivers and
> > noticed that for a fair lot we're very to be completely struct_mutex free.
> > 
> > This pile here is the the simple part, which mostly just removes code and
> > mutex_lock/unlock calls. All the patches here are independent and can be merged
> > in any order whatsoever. My plan is to send out a pull request for all those not
> > picked up by driver maintainers in 2-3 weeks or so, assuming no one complains.
> > 
> > Of course review & comments still very much welcome.
> > 
> > The more tricky 2nd part of this (and that one's not yet done) is to rework the
> > gem mmap handler to use the same kref_get_unless_zero trick as ttm. With that
> > there's no core requirement to hold struct_mutex over the final unref, which
> > means we can make that one lockless. I plan to add a gem_object_free_unlocked
> > for all the drivers which don't have any need for this lock.
> > 
> > Also there's a few more drivers which can be made struct_mutex free easily, I'll
> > propably stitch together poc patches for those.
> There's a concurrency bug in Tegra DRM currently because we don't lock
> accesses to drm_mm (I guess this demonstrates how badly we need better
> testing...) and it seems like this is typically protected by the very
> same struct_mutex that you're on a crusade to remove. If your goal is
> to get rid of it for good, should we simply add a separate lock just
> for the drm_mm? We don't have another one that would fit.

Yes, please just add your own driver-private lock. The next struct_mutex
crusade series will actually do exactly that, at least for simple cases
like armada. With that changes most drivers don't care about struct_mutex
any more in their ->gem_free_object hook and I plan to add a new
->gem_free_object_unlocked variant for all the drivers which don't have
any residual usage of struct_mutex.
Daniel Vetter
Software Engineer, Intel Corporation

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