[PATCH 02/11] drm/i915: Release ctx->syncobj on final put, not on ctx close

[Daniel Vetter]

gem context refcounting is another exercise in least locking design it
seems, where most things get destroyed upon context closure (which can
race with anything really). Only the actual memory allocation and the
locks survive while holding a reference.

This tripped up Jason when reimplementing the single timeline feature
in

commit 00dae4d3d35d4f526929633b76e00b0ab4d3970d
Author: Jason Ekstrand <jason at jlekstrand.net>
Date:   Thu Jul 8 10:48:12 2021 -0500

    drm/i915: Implement SINGLE_TIMELINE with a syncobj (v4)

We could fix the bug by holding ctx->mutex in execbuf and clear the
pointer (again while holding the mutex) context_close, but it's
cleaner to just make the context object actually invariant over its
_entire_ lifetime. This way any other ioctl that's potentially racing,
but holding a full reference, can still rely on ctx->syncobj being
an immutable pointer. Which without this change, is not the case.

Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
Fixes: 00dae4d3d35d ("drm/i915: Implement SINGLE_TIMELINE with a syncobj (v4)")
Cc: Jason Ekstrand <jason at jlekstrand.net>
Cc: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
Cc: Matthew Brost <matthew.brost at intel.com>
Cc: Matthew Auld <matthew.auld at intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: "Thomas Hellström" <thomas.hellstrom at intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
Cc: Dave Airlie <airlied at redhat.com>
---
 drivers/gpu/drm/i915/gem/i915_gem_context.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 051bc357ff65..5a053cf14948 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -994,6 +994,9 @@ static void i915_gem_context_release_work(struct work_struct *work)
 	trace_i915_context_free(ctx);
 	GEM_BUG_ON(!i915_gem_context_is_closed(ctx));
 
+	if (ctx->syncobj)
+		drm_syncobj_put(ctx->syncobj);
+
 	mutex_destroy(&ctx->engines_mutex);
 	mutex_destroy(&ctx->lut_mutex);
 
@@ -1220,9 +1223,6 @@ static void context_close(struct i915_gem_context *ctx)
 	if (vm)
 		i915_vm_close(vm);
 
-	if (ctx->syncobj)
-		drm_syncobj_put(ctx->syncobj);
-
 	ctx->file_priv = ERR_PTR(-EBADF);
 
 	/*
-- 
2.32.0