drm/i915: Do not hold mutex when faulting in user addresses

Linus Torvalds found that it was rather trivial to trigger a system
freeze:

In fact, with lockdep, I don't even need to do the sysrq-d thing: it
shows the bug as it happens. It's the X server taking the same lock
recursively.

Here's the problem:

=============================================
[ INFO: possible recursive locking detected ]
2.6.37-rc2-00012-gbdbd01a #7
---------------------------------------------
Xorg/2816 is trying to acquire lock:
(&dev->struct_mutex){+.+.+.}, at: [<ffffffff812c626c>] i915_gem_fault+0x50/0x17e

but task is already holding lock:
(&dev->struct_mutex){+.+.+.}, at: [<ffffffff812c403b>] i915_mutex_lock_interruptible+0x28/0x4a

other info that might help us debug this:
2 locks held by Xorg/2816:
#0: (&dev->struct_mutex){+.+.+.}, at: [<ffffffff812c403b>] i915_mutex_lock_interruptible+0x28/0x4a
#1: (&mm->mmap_sem){++++++}, at: [<ffffffff81022d4f>] page_fault+0x156/0x37b

This recursion was introduced by rearranging the locking to avoid the
double locking on the fast path (4f27b5d and fbd5a26d) and the
introduction of the prefault to encourage the fast paths (b5e4f2b). In
order to undo the problem, we rearrange the code to perform the access
validation upfront, attempt to prefault and then fight for control of the
mutex. the best case scenario where the mutex is uncontended the
prefaulting is not wasted.

Reported-and-tested-by: Linus Torvalds <[e-mail]>
Signed-off-by: Chris Wilson <[e-mail]>

drivers/gpu/drm/i915/i915_gem.c

···
 	struct drm_i915_gem_object *obj_priv;
 	int ret = 0;
 
+	if (args->size == 0)
+		return 0;
+
+	if (!access_ok(VERIFY_WRITE,
+		       (char __user *)(uintptr_t)args->data_ptr,
+		       args->size))
+		return -EFAULT;
+
+	ret = fault_in_pages_writeable((char __user *)(uintptr_t)args->data_ptr,
+				       args->size);
+	if (ret)
+		return -EFAULT;
+
 	ret = i915_mutex_lock_interruptible(dev);
 	if (ret)
 		return ret;
···
 	/* Bounds check source.  */
 	if (args->offset > obj->size || args->size > obj->size - args->offset) {
 		ret = -EINVAL;
-		goto out;
-	}
-
-	if (args->size == 0)
-		goto out;
-
-	if (!access_ok(VERIFY_WRITE,
-		       (char __user *)(uintptr_t)args->data_ptr,
-		       args->size)) {
-		ret = -EFAULT;
-		goto out;
-	}
-
-	ret = fault_in_pages_writeable((char __user *)(uintptr_t)args->data_ptr,
-				       args->size);
-	if (ret) {
-		ret = -EFAULT;
 		goto out;
 	}
 
···
 	struct drm_i915_gem_pwrite *args = data;
 	struct drm_gem_object *obj;
 	struct drm_i915_gem_object *obj_priv;
-	int ret = 0;
+	int ret;
+
+	if (args->size == 0)
+		return 0;
+
+	if (!access_ok(VERIFY_READ,
+		       (char __user *)(uintptr_t)args->data_ptr,
+		       args->size))
+		return -EFAULT;
+
+	ret = fault_in_pages_readable((char __user *)(uintptr_t)args->data_ptr,
+				      args->size);
+	if (ret)
+		return -EFAULT;
 
 	ret = i915_mutex_lock_interruptible(dev);
 	if (ret)
···
 	}
 	obj_priv = to_intel_bo(obj);
 
-
 	/* Bounds check destination. */
 	if (args->offset > obj->size || args->size > obj->size - args->offset) {
 		ret = -EINVAL;
-		goto out;
-	}
-
-	if (args->size == 0)
-		goto out;
-
-	if (!access_ok(VERIFY_READ,
-		       (char __user *)(uintptr_t)args->data_ptr,
-		       args->size)) {
-		ret = -EFAULT;
-		goto out;
-	}
-
-	ret = fault_in_pages_readable((char __user *)(uintptr_t)args->data_ptr,
-				      args->size);
-	if (ret) {
-		ret = -EFAULT;
 		goto out;
 	}