Re: [PATCH 08/53] drm/i915: Update alloc_request to return the allocated request

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On 05/03/2015 15:27, Tomas Elf wrote:
On 19/02/2015 17:17, John.C.Harrison@xxxxxxxxx wrote:
From: John Harrison <John.C.Harrison@xxxxxxxxx>

The alloc_request() function does not actually return the newly allocated request. Instead, it must be pulled from ring->outstanding_lazy_request. This patch fixes this so that code can create a request and start using it knowing
exactly which request it actually owns.

For: VIZ-5115
Signed-off-by: John Harrison <John.C.Harrison@xxxxxxxxx>
---
  drivers/gpu/drm/i915/i915_drv.h            |    3 ++-
  drivers/gpu/drm/i915/i915_gem_execbuffer.c |    3 ++-
  drivers/gpu/drm/i915/intel_lrc.c           |   13 +++++++++----
  drivers/gpu/drm/i915/intel_lrc.h           |    3 ++-
  drivers/gpu/drm/i915/intel_ringbuffer.c    |   14 ++++++++++----
  drivers/gpu/drm/i915/intel_ringbuffer.h    |    3 ++-
  6 files changed, 27 insertions(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 87a4a2e..90223f208 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1909,7 +1909,8 @@ struct drm_i915_private {
/* Abstract the submission mechanism (legacy ringbuffer or execlists) away */
      struct {
          int (*alloc_request)(struct intel_engine_cs *ring,
-                     struct intel_context *ctx);
+                     struct intel_context *ctx,
+                     struct drm_i915_gem_request **req_out);
          int (*do_execbuf)(struct i915_execbuffer_params *params,
                    struct drm_i915_gem_execbuffer2 *args,
                    struct list_head *vmas);
diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index 61471e9..37dcc6f 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -1353,6 +1353,7 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
      struct i915_address_space *vm;
struct i915_execbuffer_params params_master; /* XXX: will be removed later */
      struct i915_execbuffer_params *params = &params_master;
+    struct drm_i915_gem_request *request;

Please initialize request to NULL. If you accidentally dereference it before it is allocated (seeing as the allocation is several pages down from here) you get a null pointer exception, which is a clear indication that you did something stupid. Otherwise it's not clear what will happen (sure, page fault, but null pointer exception is a better failure indication).

That should generate a 'use before assignment' compiler warning.


Thanks,
Tomas

      const u32 ctx_id = i915_execbuffer2_get_context_id(*args);
      u32 dispatch_flags;
      int ret;
@@ -1531,7 +1532,7 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data, params->batch_obj_vm_offset = i915_gem_obj_offset(batch_obj, vm);

      /* Allocate a request for this batch buffer nice and early. */
-    ret = dev_priv->gt.alloc_request(ring, ctx);
+    ret = dev_priv->gt.alloc_request(ring, ctx, &request);
      if (ret)
          goto err;

diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
index 8628abf..c3c783f 100644
--- a/drivers/gpu/drm/i915/intel_lrc.c
+++ b/drivers/gpu/drm/i915/intel_lrc.c
@@ -857,13 +857,17 @@ void intel_lr_context_unpin(struct intel_engine_cs *ring,
  }

  int intel_logical_ring_alloc_request(struct intel_engine_cs *ring,
-                     struct intel_context *ctx)
+                     struct intel_context *ctx,
+                     struct drm_i915_gem_request **req_out)
  {
      struct drm_i915_gem_request *request;
      struct drm_i915_private *dev_private = ring->dev->dev_private;
      int ret;

-    if (ring->outstanding_lazy_request)
+    if (!req_out)
+        return -EINVAL;
+
+    if ((*req_out = ring->outstanding_lazy_request) != NULL)
          return 0;

      request = kzalloc(sizeof(*request), GFP_KERNEL);
@@ -898,7 +902,7 @@ int intel_logical_ring_alloc_request(struct intel_engine_cs *ring,
      i915_gem_context_reference(request->ctx);
      request->ringbuf = ctx->engine[ring->id].ringbuf;

-    ring->outstanding_lazy_request = request;
+    *req_out = ring->outstanding_lazy_request = request;
      return 0;
  }

@@ -1051,6 +1055,7 @@ static int logical_ring_prepare(struct intel_ringbuffer *ringbuf,
  int intel_logical_ring_begin(struct intel_ringbuffer *ringbuf,
                   struct intel_context *ctx, int num_dwords)
  {
+    struct drm_i915_gem_request *req;
      struct intel_engine_cs *ring = ringbuf->ring;
      struct drm_device *dev = ring->dev;
      struct drm_i915_private *dev_priv = dev->dev_private;
@@ -1066,7 +1071,7 @@ int intel_logical_ring_begin(struct intel_ringbuffer *ringbuf,
          return ret;

      /* Preallocate the olr before touching the ring */
-    ret = intel_logical_ring_alloc_request(ring, ctx);
+    ret = intel_logical_ring_alloc_request(ring, ctx, &req);
      if (ret)
          return ret;

diff --git a/drivers/gpu/drm/i915/intel_lrc.h b/drivers/gpu/drm/i915/intel_lrc.h
index 3cc38bd..77de8ac 100644
--- a/drivers/gpu/drm/i915/intel_lrc.h
+++ b/drivers/gpu/drm/i915/intel_lrc.h
@@ -37,7 +37,8 @@

  /* Logical Rings */
int __must_check intel_logical_ring_alloc_request(struct intel_engine_cs *ring,
-                          struct intel_context *ctx);
+                          struct intel_context *ctx,
+                          struct drm_i915_gem_request **req_out);
  void intel_logical_ring_stop(struct intel_engine_cs *ring);
  void intel_logical_ring_cleanup(struct intel_engine_cs *ring);
  int intel_logical_rings_init(struct drm_device *dev);
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 635707a..1a9f884 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -2164,13 +2164,18 @@ int intel_ring_idle(struct intel_engine_cs *ring)
  }

  int
-intel_ring_alloc_request(struct intel_engine_cs *ring, struct intel_context *ctx)
+intel_ring_alloc_request(struct intel_engine_cs *ring,
+             struct intel_context *ctx,
+             struct drm_i915_gem_request **req_out)
  {
      int ret;
      struct drm_i915_gem_request *request;
      struct drm_i915_private *dev_private = ring->dev->dev_private;

-    if (ring->outstanding_lazy_request)
+    if (!req_out)
+        return -EINVAL;
+
+    if ((*req_out = ring->outstanding_lazy_request) != NULL)
          return 0;

      request = kzalloc(sizeof(*request), GFP_KERNEL);
@@ -2188,7 +2193,7 @@ intel_ring_alloc_request(struct intel_engine_cs *ring, struct intel_context *ctx
          return ret;
      }

-    ring->outstanding_lazy_request = request;
+    *req_out = ring->outstanding_lazy_request = request;
      return 0;
  }

@@ -2216,6 +2221,7 @@ static int __intel_ring_prepare(struct intel_engine_cs *ring,
  int intel_ring_begin(struct intel_engine_cs *ring,
               int num_dwords)
  {
+    struct drm_i915_gem_request *req;
      struct drm_i915_private *dev_priv = ring->dev->dev_private;
      int ret;

@@ -2229,7 +2235,7 @@ int intel_ring_begin(struct intel_engine_cs *ring,
          return ret;

      /* Preallocate the olr before touching the ring */
-    ret = intel_ring_alloc_request(ring, NULL);
+    ret = intel_ring_alloc_request(ring, NULL, &req);
      if (ret)
          return ret;

diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 2fd960a..4f8a14a 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -393,7 +393,8 @@ void intel_cleanup_ring_buffer(struct intel_engine_cs *ring); int __must_check intel_ring_begin(struct intel_engine_cs *ring, int n); int __must_check intel_ring_cacheline_align(struct intel_engine_cs *ring); int __must_check intel_ring_alloc_request(struct intel_engine_cs *ring,
-                      struct intel_context *ctx);
+                      struct intel_context *ctx,
+                      struct drm_i915_gem_request **req_out);
  static inline void intel_ring_emit(struct intel_engine_cs *ring,
                     u32 data)
  {



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