From 30fe7b07f6cb7f391181eb9d2ea2687cd9bcf395 Mon Sep 17 00:00:00 2001
From: Chris Wilson <chris@chris-wilson.co.uk>
Date: Fri, 12 Jul 2019 09:03:14 +0100
Subject: dma-buf: Relax the write-seqlock for reallocating the shared fence
list
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Git-commit: 30fe7b07f6cb7f391181eb9d2ea2687cd9bcf395
Patch-mainline: v5.4-rc1
References: bsc#1152472
As the set of shared fences is not being changed during reallocation of
the reservation list, we can skip updating the write_seqlock.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712080314.21018-2-chris@chris-wilson.co.uk
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
---
drivers/dma-buf/reservation.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)
diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
index 80ecc1283d15..c71b85c8c159 100644
--- a/drivers/dma-buf/reservation.c
+++ b/drivers/dma-buf/reservation.c
@@ -157,15 +157,15 @@ int reservation_object_reserve_shared(struct reservation_object *obj,
(ksize(new) - offsetof(typeof(*new), shared)) /
sizeof(*new->shared);
- preempt_disable();
- write_seqcount_begin(&obj->seq);
/*
- * RCU_INIT_POINTER can be used here,
- * seqcount provides the necessary barriers
+ * We are not changing the effective set of fences here so can
+ * merely update the pointer to the new array; both existing
+ * readers and new readers will see exactly the same set of
+ * active (unsignaled) shared fences. Individual fences and the
+ * old array are protected by RCU and so will not vanish under
+ * the gaze of the rcu_read_lock() readers.
*/
- RCU_INIT_POINTER(obj->fence, new);
- write_seqcount_end(&obj->seq);
- preempt_enable();
+ rcu_assign_pointer(obj->fence, new);
if (!old)
return 0;
--
2.28.0