[PATCH 14/15] dma-buf: drop seq count based update

Christian König
2022-04-07T09:00:12Z

This should be possible now since we don't have the distinction
between exclusive and shared fences any more.

The only possible pitfall is that a dma_fence would be reused during the
RCU grace period, but even that could be handled with a single extra check.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
---
drivers/dma-buf/dma-resv.c | 33 ++++++++++++---------------------
drivers/dma-buf/st-dma-resv.c | 2 +-
include/linux/dma-resv.h | 12 ------------
3 files changed, 13 insertions(+), 34 deletions(-)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 5b64aa554c36..0cce6e4ec946 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -133,7 +133,6 @@ static void dma_resv_list_free(struct dma_resv_list *list)
void dma_resv_init(struct dma_resv *obj)
{
ww_mutex_init(&obj->lock, &reservation_ww_class);
- seqcount_ww_mutex_init(&obj->seq, &obj->lock);

RCU_INIT_POINTER(obj->fences, NULL);
}
@@ -292,28 +291,24 @@ void dma_resv_add_fence(struct dma_resv *obj, struct dma_fence *fence,
fobj = dma_resv_fences_list(obj);
count = fobj->num_fences;

- write_seqcount_begin(&obj->seq);
-
for (i = 0; i < count; ++i) {
enum dma_resv_usage old_usage;

dma_resv_list_entry(fobj, i, obj, &old, &old_usage);
if ((old->context == fence->context && old_usage >= usage) ||
- dma_fence_is_signaled(old))
- goto replace;
+ dma_fence_is_signaled(old)) {
+ dma_resv_list_set(fobj, i, fence, usage);
+ dma_fence_put(old);
+ return;
+ }
}

BUG_ON(fobj->num_fences >= fobj->max_fences);
- old = NULL;
count++;

-replace:
dma_resv_list_set(fobj, i, fence, usage);
/* pointer update must be visible before we extend the num_fences */
smp_store_mb(fobj->num_fences, count);
-
- write_seqcount_end(&obj->seq);
- dma_fence_put(old);
}
EXPORT_SYMBOL(dma_resv_add_fence);

@@ -341,7 +336,6 @@ void dma_resv_replace_fences(struct dma_resv *obj, uint64_t context,
dma_resv_assert_held(obj);

list = dma_resv_fences_list(obj);
- write_seqcount_begin(&obj->seq);
for (i = 0; list && i < list->num_fences; ++i) {
struct dma_fence *old;

@@ -352,14 +346,12 @@ void dma_resv_replace_fences(struct dma_resv *obj, uint64_t context,
dma_resv_list_set(list, i, replacement, usage);
dma_fence_put(old);
}
- write_seqcount_end(&obj->seq);
}
EXPORT_SYMBOL(dma_resv_replace_fences);

/* Restart the unlocked iteration by initializing the cursor object. */
static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
{
- cursor->seq = read_seqcount_begin(&cursor->obj->seq);
cursor->index = 0;
cursor->num_fences = 0;
cursor->fences = dma_resv_fences_list(cursor->obj);
@@ -388,8 +380,10 @@ static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor)
cursor->obj, &cursor->fence,
&cursor->fence_usage);
cursor->fence = dma_fence_get_rcu(cursor->fence);
- if (!cursor->fence)
- break;
+ if (!cursor->fence) {
+ dma_resv_iter_restart_unlocked(cursor);
+ continue;
+ }

if (!dma_fence_is_signaled(cursor->fence) &&
cursor->usage >= cursor->fence_usage)
@@ -415,7 +409,7 @@ struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor)
do {
dma_resv_iter_restart_unlocked(cursor);
dma_resv_iter_walk_unlocked(cursor);
- } while (read_seqcount_retry(&cursor->obj->seq, cursor->seq));
+ } while (dma_resv_fences_list(cursor->obj) != cursor->fences);
rcu_read_unlock();

return cursor->fence;
@@ -438,13 +432,13 @@ struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor)

rcu_read_lock();
cursor->is_restarted = false;
- restart = read_seqcount_retry(&cursor->obj->seq, cursor->seq);
+ restart = dma_resv_fences_list(cursor->obj) != cursor->fences;
do {
if (restart)
dma_resv_iter_restart_unlocked(cursor);
dma_resv_iter_walk_unlocked(cursor);
restart = true;
- } while (read_seqcount_retry(&cursor->obj->seq, cursor->seq));
+ } while (dma_resv_fences_list(cursor->obj) != cursor->fences);
rcu_read_unlock();

return cursor->fence;
@@ -540,10 +534,7 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
}
dma_resv_iter_end(&cursor);

- write_seqcount_begin(&dst->seq);
list = rcu_replace_pointer(dst->fences, list, dma_resv_held(dst));
- write_seqcount_end(&dst->seq);
-
dma_resv_list_free(list);
return 0;
}
diff --git a/drivers/dma-buf/st-dma-resv.c b/drivers/dma-buf/st-dma-resv.c
index 8ace9e84c845..813779e3c9be 100644
--- a/drivers/dma-buf/st-dma-resv.c
+++ b/drivers/dma-buf/st-dma-resv.c
@@ -217,7 +217,7 @@ static int test_for_each_unlocked(void *arg)
if (r == -ENOENT) {
r = -EINVAL;
/* That should trigger an restart */
- cursor.seq--;
+ cursor.fences = (void*)~0;
} else if (r == -EINVAL) {
r = 0;
}
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index 1db759eacc98..c8ccbc94d5d2 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -155,15 +155,6 @@ struct dma_resv {
*/
struct ww_mutex lock;

- /**
- * @seq:
- *
- * Sequence count for managing RCU read-side synchronization, allows
- * read-only access to @fences while ensuring we take a consistent
- * snapshot.
- */
- seqcount_ww_mutex_t seq;
-
/**
* @fences:
*
@@ -202,9 +193,6 @@ struct dma_resv_iter {
/** @fence_usage: the usage of the current fence */
enum dma_resv_usage fence_usage;

- /** @seq: sequence number to check for modifications */
- unsigned int seq;
-
/** @index: index into the shared fences */
unsigned int index;

--
2.25.1