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.
Thierry