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dma-fence: Store the timestamp in the same union as the cb_list

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The timestamp and the cb_list are mutually exclusive, the cb_list can
only be added to prior to being signaled (and once signaled we drain),
while the timestamp is only valid upon being signaled. Both the
timestamp and the cb_list are only valid while the fence is alive, and
as soon as no references are held can be replaced by the rcu_head.

By reusing the union for the timestamp, we squeeze the base dma_fence
struct to 64 bytes on x86-64.

v2: Sort the union chronologically

Suggested-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>.
Link: https://patchwork.freedesktop.org/patch/msgid/20190817153022.5749-1-chris@chris-wilson.co.uk
This commit is contained in:
Chris Wilson
2019-08-17 16:30:22 +01:00
parent 0fc89b6802
commit f2cb60e9a3
4 changed files with 38 additions and 18 deletions
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24
include/linux/dma-fence.h
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@@ -65,17 +65,31 @@ struct dma_fence_cb;
struct dma_fence {
spinlock_t *lock;
const struct dma_fence_ops *ops;
/* We clear the callback list on kref_put so that by the time we
* release the fence it is unused. No one should be adding to the cb_list
* that they don't themselves hold a reference for.
/*
* We clear the callback list on kref_put so that by the time we
* release the fence it is unused. No one should be adding to the
* cb_list that they don't themselves hold a reference for.
*
* The lifetime of the timestamp is similarly tied to both the
* rcu freelist and the cb_list. The timestamp is only set upon
* signaling while simultaneously notifying the cb_list. Ergo, we
* only use either the cb_list of timestamp. Upon destruction,
* neither are accessible, and so we can use the rcu. This means
* that the cb_list is *only* valid until the signal bit is set,
* and to read either you *must* hold a reference to the fence,
* and not just the rcu_read_lock.
*
* Listed in chronological order.
*/
union {
struct rcu_head rcu;
struct list_head cb_list;
/* @cb_list replaced by @timestamp on dma_fence_signal() */
ktime_t timestamp;
/* @timestamp replaced by @rcu on dma_fence_release() */
struct rcu_head rcu;
};
u64 context;
u64 seqno;
ktime_t timestamp;
unsigned long flags;
struct kref refcount;
int error;