[Intel-gfx] [PATCH 03/11] drm/i915: Keep gem ctx->vm alive until the final put

Daniel Vetter
2021-08-13T20:31:14Z

The comment added in

commit b81dde719439c8f09bb61e742ed95bfc4b33946b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date: Tue May 21 22:11:29 2019 +0100

drm/i915: Allow userspace to clone contexts on creation

and moved in

commit 27dbae8f36c1c25008b7885fc07c57054b7dfba3
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date: Wed Nov 6 09:13:12 2019 +0000

drm/i915/gem: Safely acquire the ctx->vm when copying

suggested that i915_address_space were at least intended to be managed
through SLAB_TYPESAFE_BY_RCU:

* This ppgtt may have be reallocated between
* the read and the kref, and reassigned to a third
* context. In order to avoid inadvertent sharing
* of this ppgtt with that third context (and not
* src), we have to confirm that we have the same
* ppgtt after passing through the strong memory
* barrier implied by a successful
* kref_get_unless_zero().

But extensive git history search has not brough any such reuse to
light.

What has come to light though is that ever since

commit 2850748ef8763ab46958e43a4d1c445f29eeb37d
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date: Fri Oct 4 14:39:58 2019 +0100

drm/i915: Pull i915_vma_pin under the vm->mutex

(yes this commit is earlier) the final i915_vma_put call has been
moved from i915_gem_context_free (now called _release) to
context_close, which means it's not actually safe anymore to access
the ctx->vm pointer without lock helds, because it might disappear at
any moment. Note that superficially things all still work, because the
i915_address_space is RCU protected since

commit b32fa811156328aea5a3c2ff05cc096490382456
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date: Thu Jun 20 19:37:05 2019 +0100

drm/i915/gtt: Defer address space cleanup to an RCU worker

except the very clever macro above (which is designed to protected
against object reuse due to SLAB_TYPESAFE_BY_RCU or similar tricks)
results in an endless loop if the refcount of the ctx->vm ever
permanently drops to 0. Which it totally now can.

Fix that by moving the final i915_vm_put to where it should be.

Note that i915_gem_context is rcu protected, but _only_ the final
kfree. This means anyone who chases a pointer to a gem ctx solely
under the protection can pretty only call kref_get_unless_zero(). This
seems to be pretty much the case, aside from a bunch of cases that
consult the scheduling information without any further protection.

Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: "Thomas Hellström" <thomas.hellstrom@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Fixes: 2850748ef876 ("drm/i915: Pull i915_vma_pin under the vm->mutex")
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
drivers/gpu/drm/i915/gem/i915_gem_context.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 5a053cf14948..12e2de1db1a2 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -990,6 +990,7 @@ static void i915_gem_context_release_work(struct work_struct *work)
{
struct i915_gem_context *ctx = container_of(work, typeof(*ctx),
release_work);
+ struct i915_address_space *vm;

trace_i915_context_free(ctx);
GEM_BUG_ON(!i915_gem_context_is_closed(ctx));
@@ -997,6 +998,10 @@ static void i915_gem_context_release_work(struct work_struct *work)
if (ctx->syncobj)
drm_syncobj_put(ctx->syncobj);

+ vm = i915_gem_context_vm(ctx);
+ if (vm)
+ i915_vm_put(vm);
+
mutex_destroy(&ctx->engines_mutex);
mutex_destroy(&ctx->lut_mutex);

@@ -1220,8 +1225,15 @@ static void context_close(struct i915_gem_context *ctx)
set_closed_name(ctx);

vm = i915_gem_context_vm(ctx);
- if (vm)
+ if (vm) {
+ /* i915_vm_close drops the final reference, which is a bit too
+ * early and could result in surprises with concurrent
+ * operations racing with thist ctx close. Keep a full reference
+ * until the end.
+ */
+ i915_vm_get(vm);
i915_vm_close(vm);
+ }

ctx->file_priv = ERR_PTR(-EBADF);

--
2.32.0