RFC
I don't have a good name for this yet and I did not spend any time on documentataion (for that reason)
We create fences (out fences) as part of operations execution, which are short-lived objects, we want to release all memory after operation execution is completed or when operation gets cancelled/deleted via ioctl().
This creates a bit of a problem. DMA fences are refcounted objects and exporter never knows when importer imports a fence or puts its refcount, so exporter never knows when fence will be destoyed, which should not be a problem for refcounted objects, but here comes the twist...
operation A - creates and exports out fence X ... user-space imports fence X operation A - finishes execution, signals fence X kfree operation A, put dma_fence
DMA fences are designed to borrow spinlock that DMA fences use to protect struct dma_fence members:
struct dma_fence { spinlock_t *lock;
const struct dma_fence_ops *ops; ..... };
void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, spinlock_t *lock, u64 context, u64 seqno);
So the `lock` should have at least same lifespan as the DMA fence that borrows it, which is impossible to guarantee in our case. When we kfree operation A struct we also kfree ->lock that operation lends to DMA fence, which outlives operation A (depending on what fence importers do and when they drop imported fence refcount).
This patch adds a new memnber to struct dma_fence: __lock_inplace. Which is a lock that DMA fence will use to protect its own data when it cannot reliably borrow a lock from the outside object.
I also had a patch that puts inplace and borrowed locks to an unnamed uninon and adds one more dma_fence_flag_bits to distinguish between fences with borrowed and inplace locks
struct dma_fence { uninon { spinlock_t *lock; spinlock_t __lock_inplace; }; ... };
And then instead of locking/unlocking ->lock directly we would use dma_fence_lock_irqsave()/dma_fence_unlock_irqrestore() macros which would check fence flags and either use borrowed lock or inplace lock. But after seeing how owten drivers directly access fence ->lock I decided to scratch that approach and just add extra spinlock member.
Not-Yet-Signed-off-by: Sergey Senozhatsky senozhatsky@chromium.org --- drivers/dma-buf/dma-fence.c | 10 ++++++++++ include/linux/dma-fence.h | 6 ++++++ 2 files changed, 16 insertions(+)
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index 066400ed8841..7ae40b8adb73 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -958,3 +958,13 @@ dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, trace_dma_fence_init(fence); } EXPORT_SYMBOL(dma_fence_init); + +void dma_fence_inplace_lock_init(struct dma_fence *fence, + const struct dma_fence_ops *ops, + u64 context, u64 seqno) +{ + spin_lock_init(&fence->__lock_inplace); + + dma_fence_init(fence, ops, &fence->__lock_inplace, context, seqno); +} +EXPORT_SYMBOL(dma_fence_inplace_lock_init); diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h index 1ea691753bd3..6b15a0d2eccf 100644 --- a/include/linux/dma-fence.h +++ b/include/linux/dma-fence.h @@ -64,6 +64,8 @@ struct dma_fence_cb; */ struct dma_fence { spinlock_t *lock; + spinlock_t __lock_inplace; + const struct dma_fence_ops *ops; /* * We clear the callback list on kref_put so that by the time we @@ -262,6 +264,10 @@ struct dma_fence_ops { void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops, spinlock_t *lock, u64 context, u64 seqno);
+void dma_fence_inplace_lock_init(struct dma_fence *fence, + const struct dma_fence_ops *ops, + u64 context, u64 seqno); + void dma_fence_release(struct kref *kref); void dma_fence_free(struct dma_fence *fence); void dma_fence_describe(struct dma_fence *fence, struct seq_file *seq);