On Mon, 2025-10-13 at 15:48 +0200, Christian König wrote:
> When neither a release nor a wait operation is specified it is possible
> to let the dma_fence live on independent of the module who issued it.
>
> This makes it possible to unload drivers and only wait for all their
> fences to signal.
>
> Signed-off-by: Christian König <[email protected]>
> ---
> drivers/dma-buf/dma-fence.c | 16 ++++++++++++----
> include/linux/dma-fence.h | 4 ++--
> 2 files changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 982f2b2a62c0..39f73edf3a33 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -374,6 +374,14 @@ int dma_fence_signal_timestamp_locked(struct dma_fence
> *fence,
> &fence->flags)))
> return -EINVAL;
>
> + /*
> + * When neither a release nor a wait operation is specified set the ops
> + * pointer to NULL to allow the fence structure to become independent
> + * who originally issued it.
> + */
> + if (!fence->ops->release && !fence->ops->wait)
> + RCU_INIT_POINTER(fence->ops, NULL);
OK, so the basic idea is that still living fences can't access driver
data or driver code anymore after the driver is unloaded. Good and
well, nice idea. We need something like that in Rust, too.
That's based on the rule that the driver, on unload, must signal all
the fences. Also OK.
However, how can that possibly fly by relying on the release callback
not being implemented? How many users don't need it, and could those
who implement release() be ported to.. sth else?
P.
> +
> /* Stash the cb_list before replacing it with the timestamp */
> list_replace(&fence->cb_list, &cb_list);
>
> @@ -513,7 +521,7 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool
> intr, signed long timeout)
> rcu_read_lock();
> ops = rcu_dereference(fence->ops);
> trace_dma_fence_wait_start(fence);
> - if (ops->wait) {
> + if (ops && ops->wait) {
> /*
> * Implementing the wait ops is deprecated and not supported for
> * issuer independent fences, so it is ok to use the ops outside
> @@ -578,7 +586,7 @@ void dma_fence_release(struct kref *kref)
> }
>
> ops = rcu_dereference(fence->ops);
> - if (ops->release)
> + if (ops && ops->release)
> ops->release(fence);
> else
> dma_fence_free(fence);
> @@ -614,7 +622,7 @@ static bool __dma_fence_enable_signaling(struct dma_fence
> *fence)
>
> rcu_read_lock();
> ops = rcu_dereference(fence->ops);
> - if (!was_set && ops->enable_signaling) {
> + if (!was_set && ops && ops->enable_signaling) {
> trace_dma_fence_enable_signal(fence);
>
> if (!ops->enable_signaling(fence)) {
> @@ -1000,7 +1008,7 @@ void dma_fence_set_deadline(struct dma_fence *fence,
> ktime_t deadline)
>
> rcu_read_lock();
> ops = rcu_dereference(fence->ops);
> - if (ops->set_deadline && !dma_fence_is_signaled(fence))
> + if (ops && ops->set_deadline && !dma_fence_is_signaled(fence))
> ops->set_deadline(fence, deadline);
> rcu_read_unlock();
> }
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index 38421a0c7c5b..e1ba1d53de88 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -425,7 +425,7 @@ dma_fence_is_signaled_locked(struct dma_fence *fence)
>
> rcu_read_lock();
> ops = rcu_dereference(fence->ops);
> - if (ops->signaled && ops->signaled(fence)) {
> + if (ops && ops->signaled && ops->signaled(fence)) {
> rcu_read_unlock();
> dma_fence_signal_locked(fence);
> return true;
> @@ -461,7 +461,7 @@ dma_fence_is_signaled(struct dma_fence *fence)
>
> rcu_read_lock();
> ops = rcu_dereference(fence->ops);
> - if (ops->signaled && ops->signaled(fence)) {
> + if (ops && ops->signaled && ops->signaled(fence)) {
> rcu_read_unlock();
> dma_fence_signal(fence);
> return true;
