[PATCH] 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, but it's cleaner to just
make the context object actually invariant over its _entire_ lifetime.

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 754b9b8d4981..93ba0197d70a 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -940,6 +940,9 @@ void i915_gem_context_release(struct kref *ref)
 	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);
 
@@ -1159,9 +1162,6 @@ 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