drm/i915: Use a ctor for TYPESAFE_BY_RCU i915_request

drm/i915: Use a ctor for TYPESAFE_BY_RCU i915_request

As we start peeking into requests for longer and longer, e.g.
incorporating use of spinlocks when only protected by an
rcu_read_lock(), we need to be careful in how we reset the request when
recycling and need to preserve any barriers that may still be in use as
the request is reset for reuse.

Quoting Linus Torvalds:

> If there is refcounting going on then why use SLAB_TYPESAFE_BY_RCU?

  .. because the object can be accessed (by RCU) after the refcount has
  gone down to zero, and the thing has been released.

  That's the whole and only point of SLAB_TYPESAFE_BY_RCU.

  That flag basically says:

  "I may end up accessing this object *after* it has been free'd,
  because there may be RCU lookups in flight"

  This has nothing to do with constructors. It's ok if the object gets
  reused as an object of the same type and does *not* get
  re-initialized, because we're perfectly fine seeing old stale data.

  What it guarantees is that the slab isn't shared with any other kind
  of object, _and_ that the underlying pages are free'd after an RCU
  quiescent period (so the pages aren't shared with another kind of
  object either during an RCU walk).

  And it doesn't necessarily have to have a constructor, because the
  thing that a RCU walk will care about is

    (a) guaranteed to be an object that *has* been on some RCU list (so
    it's not a "new" object)

    (b) the RCU walk needs to have logic to verify that it's still the
    *same* object and hasn't been re-used as something else.

  In contrast, a SLAB_TYPESAFE_BY_RCU memory gets free'd and re-used
  immediately, but because it gets reused as the same kind of object,
  the RCU walker can "know" what parts have meaning for re-use, in a way
  it couidn't if the re-use was random.

  That said, it *is* subtle, and people should be careful.

> So the re-use might initialize the fields lazily, not necessarily using a ctor.

  If you have a well-defined refcount, and use "atomic_inc_not_zero()"
  to guard the speculative RCU access section, and use
  "atomic_dec_and_test()" in the freeing section, then you should be
  safe wrt new allocations.

  If you have a completely new allocation that has "random stale
  content", you know that it cannot be on the RCU list, so there is no
  speculative access that can ever see that random content.

  So the only case you need to worry about is a re-use allocation, and
  you know that the refcount will start out as zero even if you don't
  have a constructor.

  So you can think of the refcount itself as always having a zero
  constructor, *BUT* you need to be careful with ordering.

  In particular, whoever does the allocation needs to then set the
  refcount to a non-zero value *after* it has initialized all the other
  fields. And in particular, it needs to make sure that it uses the
  proper memory ordering to do so.

  NOTE! One thing to be very worried about is that re-initializing
  whatever RCU lists means that now the RCU walker may be walking on the
  wrong list so the walker may do the right thing for this particular
  entry, but it may miss walking *other* entries. So then you can get
  spurious lookup failures, because the RCU walker never walked all the
  way to the end of the right list. That ends up being a much more
  subtle bug.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191122094924.629690-1-chris@chris-wilson.co.uk