[PATCH 03/52] drm: add managed resources tied to drm_device

[Greg Kroah-Hartman]

On Wed, Feb 19, 2020 at 03:28:47PM +0200, Laurent Pinchart wrote:
> Hi Daniel,
> 
> Thank you for the patch.
> 
> On Wed, Feb 19, 2020 at 11:20:33AM +0100, Daniel Vetter wrote:
> > We have lots of these. And the cleanup code tends to be of dubious
> > quality. The biggest wrong pattern is that developers use devm_, which
> > ties the release action to the underlying struct device, whereas
> > all the userspace visible stuff attached to a drm_device can long
> > outlive that one (e.g. after a hotunplug while userspace has open
> > files and mmap'ed buffers). Give people what they want, but with more
> > correctness.
> > 
> > Mostly copied from devres.c, with types adjusted to fit drm_device and
> > a few simplifications - I didn't (yet) copy over everything. Since
> > the types don't match code sharing looked like a hopeless endeavour.
> > 
> > For now it's only super simplified, no groups, you can't remove
> > actions (but kfree exists, we'll need that soon). Plus all specific to
> > drm_device ofc, including the logging. Which I didn't bother to make
> > compile-time optional, since none of the other drm logging is compile
> > time optional either.
> > 
> > One tricky bit here is the chicken&egg between allocating your
> > drm_device structure and initiliazing it with drm_dev_init. For
> > perfect onion unwinding we'd need to have the action to kfree the
> > allocation registered before drm_dev_init registers any of its own
> > release handlers. But drm_dev_init doesn't know where exactly the
> > drm_device is emebedded into the overall structure, and by the time it
> > returns it'll all be too late. And forcing drivers to be able clean up
> > everything except the one kzalloc is silly.
> > 
> > Work around this by having a very special final_kfree pointer. This
> > also avoids troubles with the list head possibly disappearing from
> > underneath us when we release all resources attached to the
> > drm_device.
> 
> This is all a very good idea ! Many subsystems are plagged by drivers
> using devm_k*alloc to allocate data accessible by userspace. Since the
> introduction of devm_*, we've likely reduced the number of memory leaks,
> but I'm pretty sure we've increased the risk of crashes as I've seen
> some drivers that used .release() callbacks correctly being naively
> converted to incorrect devm_* usage :-(
> 
> This leads me to a question: if other subsystems have the same problem,
> could we turn this implementation into something more generic ? It
> doesn't have to be done right away and shouldn't block merging this
> series, but I think it would be very useful.

It shouldn't be that hard to tie this into a drv_m() type of a thing
(driver_memory?)

And yes, I think it's much better than devm_* for the obvious reasons of
this being needed here.

thanks,

greg k-h