Re: [PATCH 2/3] drm/i915: Drop inspection of execbuf flags during evict

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Daniel Vetter (2019-11-08 10:20:23)
> On Fri, Nov 8, 2019 at 11:11 AM Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > Quoting Daniel Vetter (2019-11-08 09:54:42)
> > > On Wed, Nov 6, 2019 at 4:49 PM Chris Wilson <chris@chris-wilson.co.uk> wrote:
> > > >
> > > > With the goal of removing the serialisation from around execbuf, we will
> > > > no longer have the privilege of there being a single execbuf in flight
> > > > at any time and so will only be able to inspect the user's flags within
> > > > the carefully controlled execbuf context. i915_gem_evict_for_node() is
> > > > the only user outside of execbuf that currently peeks at the flag to
> > > > convert an overlapping softpinned request from ENOSPC to EINVAL. Retract
> > > > this nicety and only report ENOSPC if the location is in current use,
> > > > either due to this execbuf or another.
> > > >
> > > > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > > > Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > > > Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
> > >
> > > Same reasons as for patch 3, I don't think we have to do this at all.
> >
> > This is already undefined behaviour. That field is protected by
> > struct_mutex and being evaluated outside of that lock.
> 
> If this can be called on objects involved in execbuf, without
> struct_mutex, then we already have a correctness problem of vma space
> (which is super tight on old platforms and rather much required to be
> well-managed because of that) being lost because concurrent threads
> thrash it instead of forming an orderly queue. And if that's not the
> case, and they do form an orderly queue, then there's no problem since
> even the as-needed-only orderly queue provided by ww_mutex will then
> be enough locking to keep this working.

It doesn't get called on those objects, those objects may just be
neighbouring and being inspected for potential eviction candidates. The
lists themselves are protected by their mutex, it's just the contention
over the pin_count.
 
> Aside: Yeah I think we need to re-add struct_mutex to the gtt fault
> path, the temporary pinning in there could easily starve execbuf on
> platforms where batches run in ggtt. Maybe also some other areas where
> we lost struct_mutex around temporary vma->pin_count elevations.

That's where we are going next; not with struct_mutex but fenced access
to reservations to replace the temporary (not HW access) pinning.
-Chris
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