Implement asynchronous free() call

which can be called from any context where fast taskqueue are available.
Use it in i915_sw_fence code to fix #9.

drivers/gpu/drm/i915/i915_sw_fence.c

@@ -9,6 +9,11 @@
 #include <linux/irq_work.h>
 #include <linux/dma-resv.h>
 
+#ifndef __linux__
+#include <sys/param.h>		/* __FreeBSD_version */
+#include <linux/llist.h>
+#endif
+
 #include "i915_sw_fence.h"
 #include "i915_selftest.h"
 
@@ -21,6 +26,21 @@ enum {
 	DEBUG_FENCE_NOTIFY,
 };
 
+#if defined(__FreeBSD__) && __FreeBSD_version > 1300127
+static void linux_kfree_async(void *);
+/*
+ * Critical section-friendly version of kfree().
+ * Requires knowledge of the allocation size at build time.
+ */
+#define	KFREE(addr)	do {						\
+	BUILD_BUG_ON(sizeof(*(addr)) < sizeof(struct llist_node));	\
+	if (curthread->td_critnest != 0)				\
+		linux_kfree_async(addr);					\
+	else								\
+		kfree(addr);						\
+} while (0);
+#endif
+
 static void *i915_sw_fence_debug_hint(void *addr)
 {
 	return (void *)(((struct i915_sw_fence *)addr)->flags & I915_SW_FENCE_MASK);
@@ -384,7 +404,11 @@ static void dma_i915_sw_fence_wake(struct dma_fence *dma,
 
 	i915_sw_fence_set_error_once(cb->fence, dma->error);
 	i915_sw_fence_complete(cb->fence);
+#if defined(__FreeBSD__) && __FreeBSD_version > 1300127
+	KFREE(cb);
+#else
 	kfree(cb);
+#endif
 }
 
 static void timer_i915_sw_fence_wake(struct timer_list *t)
@@ -589,6 +613,27 @@ int i915_sw_fence_await_reservation(struct i915_sw_fence *fence,
 #endif
 
 #if defined(__FreeBSD__) && __FreeBSD_version > 1300127
+static struct llist_head linux_kfree_async_list = LLIST_HEAD_INIT();
+
+static void
+linux_kfree_async_fn(struct irq_work *irqw)
+{
+	struct llist_node *freed;
+
+	while((freed = llist_del_first(&linux_kfree_async_list)) != NULL)
+		kfree(freed);
+}
+static DEFINE_IRQ_WORK(linux_kfree_async_work, linux_kfree_async_fn);
+
+static void
+linux_kfree_async(void *addr)
+{
+	if (addr == NULL)
+		return;
+	llist_add(addr, &linux_kfree_async_list);
+	irq_work_queue(&linux_kfree_async_work);
+}
+
 static void
 irq_work_ctx_init_fn(struct irq_work *irqw)
 {