[src/trunk]: src/sys/external/bsd/drm2/linux Add dma_fence_get_rcu_safe.

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/6771c9d92e8e
branches:  trunk
changeset: 1027911:6771c9d92e8e
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sun Dec 19 01:15:28 2021 +0000

description:
Add dma_fence_get_rcu_safe.

diffstat:

 sys/external/bsd/drm2/include/linux/dma-fence.h |   4 +-
 sys/external/bsd/drm2/linux/linux_dma_fence.c   |  50 ++++++++++++++++++++++++-
 2 files changed, 51 insertions(+), 3 deletions(-)

diffs (89 lines):

diff -r a86d6e0d0094 -r 6771c9d92e8e sys/external/bsd/drm2/include/linux/dma-fence.h
--- a/sys/external/bsd/drm2/include/linux/dma-fence.h   Sun Dec 19 01:15:21 2021 +0000
+++ b/sys/external/bsd/drm2/include/linux/dma-fence.h   Sun Dec 19 01:15:28 2021 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: dma-fence.h,v 1.2 2021/12/19 00:27:17 riastradh Exp $  */
+/*     $NetBSD: dma-fence.h,v 1.3 2021/12/19 01:15:28 riastradh Exp $  */
 
 /*-
  * Copyright (c) 2018 The NetBSD Foundation, Inc.
@@ -112,6 +112,8 @@
        dma_fence_get(struct dma_fence *);
 struct dma_fence *
        dma_fence_get_rcu(struct dma_fence *);
+struct dma_fence *
+       dma_fence_get_rcu_safe(struct dma_fence **);
 void   dma_fence_put(struct dma_fence *);
 
 int    dma_fence_add_callback(struct dma_fence *, struct dma_fence_cb *,
diff -r a86d6e0d0094 -r 6771c9d92e8e sys/external/bsd/drm2/linux/linux_dma_fence.c
--- a/sys/external/bsd/drm2/linux/linux_dma_fence.c     Sun Dec 19 01:15:21 2021 +0000
+++ b/sys/external/bsd/drm2/linux/linux_dma_fence.c     Sun Dec 19 01:15:28 2021 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: linux_dma_fence.c,v 1.2 2021/12/19 00:27:17 riastradh Exp $    */
+/*     $NetBSD: linux_dma_fence.c,v 1.3 2021/12/19 01:15:28 riastradh Exp $    */
 
 /*-
  * Copyright (c) 2018 The NetBSD Foundation, Inc.
@@ -30,7 +30,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: linux_dma_fence.c,v 1.2 2021/12/19 00:27:17 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: linux_dma_fence.c,v 1.3 2021/12/19 01:15:28 riastradh Exp $");
 
 #include <sys/atomic.h>
 #include <sys/condvar.h>
@@ -201,6 +201,52 @@
        return fence;
 }
 
+/*
+ * dma_fence_get_rcu_safe(fencep)
+ *
+ *     Attempt to acquire a reference to the fence *fencep, which may
+ *     be about to be destroyed, during a read section.  If the value
+ *     of *fencep changes after we read *fencep but before we
+ *     increment its reference count, retry.  Return *fencep on
+ *     success, or NULL on failure.
+ */
+struct dma_fence *
+dma_fence_get_rcu_safe(struct dma_fence **fencep)
+{
+       struct dma_fence *fence, *fence0;
+
+retry:
+       fence = *fencep;
+
+       /* Load fence only once.  */
+       __insn_barrier();
+
+       /* If there's nothing there, give up.  */
+       if (fence == NULL)
+               return NULL;
+
+       /* Make sure we don't load stale fence guts.  */
+       membar_datadep_consumer();
+
+       /* Try to acquire a reference.  If we can't, try again.  */
+       if (!dma_fence_get_rcu(fence))
+               goto retry;
+
+       /*
+        * Confirm that it's still the same fence.  If not, release it
+        * and retry.
+        */
+       fence0 = *fencep;
+       __insn_barrier();
+       if (fence != fence0) {
+               dma_fence_put(fence);
+               goto retry;
+       }
+
+       /* Success!  */
+       return fence;
+}
+
 static void
 dma_fence_release(struct kref *refcount)
 {