On Mon, Aug 31, 2026 at 07:15:36PM +0530, Srinivasan Shanmugam wrote:
> GPU drivers often need to queue work when a dma-fence signals
> because certain operations (copy_to_user, eventfd_signal, memory
> allocation) cannot run in IRQ context. This pattern is currently
> open-coded in multiple drivers.
>
> Introduce drm_work_fence — an embeddable base structure that handles
> the dma-fence-callback-to-workqueue pattern in one place. Drivers
> embed this in their own structure and implement ops->work() for the
> deferred work and ops->destroy() for cleanup.
>
> The helper manages:
> - kref lifetime
> - dma-fence callback registration
> - workqueue dispatch on fence signal
> - safe cancellation before driver teardown
>
> For work that additionally requires borrowing the process MM via
> kthread_use_mm(), see drm_user_fence which builds on top of this.
>
> Suggested-by: Matthew Brost <[email protected]>
> Cc: Maarten Lankhorst <[email protected]>
> Cc: Christian König <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Signed-off-by: Srinivasan Shanmugam <[email protected]>
> ---
> drivers/gpu/drm/Makefile | 1 +
> drivers/gpu/drm/drm_work_fence.c | 195 +++++++++++++++++++++++++++++++
> include/drm/drm_work_fence.h | 76 ++++++++++++
> 3 files changed, 272 insertions(+)
> create mode 100644 drivers/gpu/drm/drm_work_fence.c
> create mode 100644 include/drm/drm_work_fence.h
>
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index e97faabcd783..c5be8e80d0c8 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -72,6 +72,7 @@ drm-y := \
> drm_vblank.o \
> drm_vblank_work.o \
> drm_vma_manager.o \
> + drm_work_fence.o \
> drm_writeback.o
> drm-$(CONFIG_DRM_CLIENT) += \
> drm_client.o \
> diff --git a/drivers/gpu/drm/drm_work_fence.c
> b/drivers/gpu/drm/drm_work_fence.c
> new file mode 100644
> index 000000000000..9f6b779d0fe9
> --- /dev/null
> +++ b/drivers/gpu/drm/drm_work_fence.c
> @@ -0,0 +1,195 @@
> +// SPDX-License-Identifier: MIT
> +/*
> + * Copyright © 2024 The Linux Foundation
> + *
> + * Common DRM work fence helper.
> + *
> + * When a GPU dma-fence signals, drivers often need to perform work that
> + * cannot run in IRQ context (e.g., memory allocation, copy_to_user,
> + * eventfd_signal). This helper queues a work item when a dma-fence
> + * signals, allowing that work to run safely in a workqueue context.
> + *
> + * NOTE: This helper consumes dma_fences but CANNOT implement
> + * dma_fence_ops. Work items queued here may sleep; dma_fence_ops
> + * callbacks are called under the fence spinlock and must not sleep.
> + *
> + * For work that additionally requires accessing userspace memory via
> + * kthread_use_mm(), see drm_user_fence which builds on top of this.
> + */
> +
> +#include <linux/workqueue.h>
> +
> +#include <drm/drm_work_fence.h>
> +
> +static void drm_work_fence_destroy(struct kref *kref)
> +{
> + struct drm_work_fence *wfence =
> + container_of(kref, struct drm_work_fence, refcount);
> +
> + if (wfence->fence)
> + dma_fence_put(wfence->fence);
> +
> + wfence->ops->destroy(wfence);
I'd invert these for safety in case destroy wants to looks at the fence,
admittedly that would be an odd use case.
So...
struct drm_work_fence *wfence =
container_of(kref, struct drm_work_fence, refcount);
struct dma_fence *fence = wfence->fence;
wfence->ops->destroy(wfence);
dma_fence_put(fence); /* this has a NULL check */
> +}
> +
> +/**
> + * drm_work_fence_get - Acquire a reference to a work fence
> + * @wfence: work fence
> + */
> +void drm_work_fence_get(struct drm_work_fence *wfence)
> +{
> + kref_get(&wfence->refcount);
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_get);
> +
> +/**
> + * drm_work_fence_put - Release a reference to a work fence
> + * @wfence: work fence
> + */
> +void drm_work_fence_put(struct drm_work_fence *wfence)
> +{
> + kref_put(&wfence->refcount, drm_work_fence_destroy);
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_put);
> +
> +static void drm_work_fence_work(struct work_struct *w)
> +{
> + struct drm_work_fence *wfence =
> + container_of(w, struct drm_work_fence, work);
> +
> + wfence->ops->work(wfence);
> + drm_work_fence_put(wfence);
> +}
> +
> +static void drm_work_fence_cb(struct dma_fence *fence, struct dma_fence_cb
> *cb)
> +{
> + struct drm_work_fence *wfence =
> + container_of(cb, struct drm_work_fence, cb);
> +
> + queue_work(wfence->wq, &wfence->work);
> + /*
> + * Put the transferred reference from add_callback. The stored
> + * reference in wfence->fence is released in drm_work_fence_destroy().
> + */
> + dma_fence_put(fence);
> +}
> +
> +/**
> + * drm_work_fence_init - Initialize a work fence
> + * @wfence: work fence to initialize
> + * @wq: workqueue to run the worker on (must be ordered if sequencing
> matters)
> + * @ops: driver operations
> + */
> +void drm_work_fence_init(struct drm_work_fence *wfence,
> + struct workqueue_struct *wq,
> + const struct drm_work_fence_ops *ops)
> +{
> + kref_init(&wfence->refcount);
> + wfence->wq = wq;
> + wfence->ops = ops;
> + wfence->fence = NULL;
> + INIT_WORK(&wfence->work, drm_work_fence_work);
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_init);
> +
> +/**
> + * drm_work_fence_add_callback - Attach a work fence to a dma-fence
> + * @wfence: work fence
> + * @fence: dma-fence to watch; ownership of this reference is transferred
> + * to the callback — caller must NOT put it afterward.
This isn't right. It is perfectly reasonable for caller to hold more
than 1 reference to @fence, thus put it again. It consumes a single
reference @fence on success or failure - that is it.
> + *
> + * When @fence signals, a work item is queued that calls ops->work().
> + * If @fence has already signaled, the work item is queued immediately.
> + *
> + * An additional reference to @fence is stored internally in @wfence to
> + * allow drm_work_fence_cancel() to be called safely without the caller
> + * needing to hold a separate fence reference.
> + *
Ideally get rid of double ref count on @fence. I don't think above
reasoning justifies the needed for a double ref on the fence. I'd tie
exactly one refernece @fence which is attached to lifetime of @wfence
(i.e., drop the dma_fence_put in drm_work_fence_cb).
> + * On any return value the caller's fence reference is consumed.
> + *
I'd mention regardless of success or fail, a reference to drm_work_fence
is consumed too.
> + * Return: 0 on success, negative errno on error.
> + */
> +int drm_work_fence_add_callback(struct drm_work_fence *wfence,
> + struct dma_fence *fence)
> +{
> + int err;
> +
> + drm_work_fence_get(wfence);
> + wfence->fence = dma_fence_get(fence);
> +
> + err = dma_fence_add_callback(fence, &wfence->cb, drm_work_fence_cb);
> + if (err == -ENOENT) {
> + queue_work(wfence->wq, &wfence->work);
> + dma_fence_put(fence);
Keep the implementation in one place?
drm_work_fence_work(&wfence->work);
> + err = 0;
> + } else if (err) {
> + dma_fence_put(wfence->fence);
> + wfence->fence = NULL;
> + drm_work_fence_put(wfence);
Won't drm_work_fence_put just drop the 'wfence->fence' reference if
'wfence->fence' isn't set to NULL. i.e., drm_work_fence_put(wfence) can
replace the above 3 lines.
> + dma_fence_put(fence);
> + }
> + /* on success: transferred ref goes to drm_work_fence_cb */
> +
> + return err;
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_add_callback);
> +
> +/**
> + * drm_work_fence_cancel - Cancel a pending work fence callback
> + * @wfence: work fence
> + *
> + * Attempts to remove the pending callback before driver context teardown.
> + * The caller must hold a reference to @wfence across this call.
> + *
> + * If the callback has already fired this returns false and all cleanup
> + * has been handled internally.
> + *
> + * If removal succeeds the callback reference is released internally.
> + * The caller must still release its own reference via drm_work_fence_put().
> + *
> + * This function is safe to call from atomic context as it only acquires
> + * the dma-fence spinlock internally. If the caller also needs to wait
> + * for the worker to finish, use drm_work_fence_cancel_sync() instead,
> + * which may sleep.
> + *
> + * Return: true if callback was removed, false if it had already fired.
> + */
> +bool drm_work_fence_cancel(struct drm_work_fence *wfence)
> +{
> + struct dma_fence *fence = wfence->fence;
> +
> + if (!fence)
> + return false;
> +
> + if (dma_fence_remove_callback(fence, &wfence->cb)) {
> + wfence->fence = NULL;
> + dma_fence_put(fence); /* callback ref */
> + dma_fence_put(fence); /* stored ref */
> + drm_work_fence_put(wfence);
Same comments as above: No need for 'wfence->fence = NULL' and
drm_work_fence_put, drm_work_fence_put is work by itself. Also see my
comment about dropped the double ref, that isn't need either.
> + return true;
> + }
> +
> + return false;
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_cancel);
> +
> +/**
> + * drm_work_fence_cancel_sync - Cancel callback and wait for worker to finish
> + * @wfence: work fence
> + *
> + * Calls drm_work_fence_cancel() then cancel_work_sync() to guarantee
> + * the worker has fully completed before returning.
> + *
> + * This function may sleep. Must not be called from atomic or interrupt
> + * context. Use drm_work_fence_cancel() instead when sleeping is not allowed.
> + *
> + * Drivers must call this during teardown before freeing any resources
> + * accessed by ops->work().
> + */
> +void drm_work_fence_cancel_sync(struct drm_work_fence *wfence)
> +{
> + drm_work_fence_cancel(wfence);
> + if (cancel_work_sync(&wfence->work))
> + drm_work_fence_put(wfence);
This will UAF if drm_work_fence_cancel removed the callback.
I actually don't think drm_work_fence_cancel, drm_work_fence_cancel_sync
is safe unless the caller has reference to drm_work_fence.
Consider the following case:
- A driver calls drm_work_fence_add_callback
- Sometime later if calls drm_work_fence_cancel or drm_work_fence_cancel_sync
- drm_work_fence_work completes before either drm_work_fence_cancel,
drm_work_fence_cancel_sync completes, we UAF
So with additional reference at the caller assumed...
I'd write this like:
if (drm_work_fence_cancel(wfence))
return; /* Worker not running, all internal refs dropped */
if (cancel_work_sync(&wfence->work))
drm_work_fence_put(wfence); /* Worker cancelled, drop it ref */
> +}
> +EXPORT_SYMBOL_GPL(drm_work_fence_cancel_sync);
> diff --git a/include/drm/drm_work_fence.h b/include/drm/drm_work_fence.h
> new file mode 100644
> index 000000000000..4fa369f937d7
> --- /dev/null
> +++ b/include/drm/drm_work_fence.h
> @@ -0,0 +1,76 @@
> +/* SPDX-License-Identifier: MIT */
> +/*
> + * Copyright © 2024 The Linux Foundation
> + */
> +
> +#ifndef __DRM_WORK_FENCE_H__
> +#define __DRM_WORK_FENCE_H__
> +
> +#include <linux/dma-fence.h>
> +#include <linux/kref.h>
> +#include <linux/workqueue.h>
> +
> +struct drm_work_fence;
> +
> +/**
> + * struct drm_work_fence_ops - driver callbacks for a DRM work fence
> + */
> +struct drm_work_fence_ops {
> + /**
> + * @work: Called from workqueue context when the dma-fence signals.
> + * Perform any work that cannot run in IRQ context here.
> + */
> + void (*work)(struct drm_work_fence *wfence);
> +
> + /**
> + * @destroy: Called when the last reference is dropped.
> + * Free the containing structure here.
> + */
> + void (*destroy)(struct drm_work_fence *wfence);
> +};
> +
> +/**
> + * struct drm_work_fence - embeddable DRM work fence
> + *
> + * Provides a dma-fence callback that queues a work item when the fence
> + * signals, allowing work that cannot run in IRQ context to be deferred
> + * to a workqueue. Drivers embed this in their own structure.
> + *
> + * NOTE: This helper is a *consumer* of dma_fences only. It CANNOT be
> + * used to implement dma_fence_ops. dma_fence callbacks are invoked
> + * while holding the fence spinlock; work queued here may sleep
> + * (copy_to_user, kthread_use_mm, eventfd_signal) and must not be
> + * called under that spinlock.
> + *
> + * Call drm_work_fence_init() at creation and drm_work_fence_add_callback()
> + * to arm. Call drm_work_fence_cancel_sync() before driver teardown.
> + */
> +struct drm_work_fence {
> + /** @refcount: Reference count. */
> + struct kref refcount;
> + /** @work: Work item queued when the dma-fence signals. */
> + struct work_struct work;
> + /** @cb: dma-fence callback. */
> + struct dma_fence_cb cb;
You could likely use union trick here on work_struct, dma_fence_cb and
only defer the INIT_WORK to drm_work_fence_cb.
> + /**
> + * @fence: Extra reference held for safe cancel(). Set during
> + * add_callback, released in destroy().
> + */
See my comments this ref count. Ideally: "A single reference held for
the lifetime of drm_work_fence after drm_work_fence_init is called"
Matt
> + struct dma_fence *fence;
> + /** @wq: Workqueue to run @work on. */
> + struct workqueue_struct *wq;
> + /** @ops: Driver operations. */
> + const struct drm_work_fence_ops *ops;
> +};
> +
> +void drm_work_fence_init(struct drm_work_fence *wfence,
> + struct workqueue_struct *wq,
> + const struct drm_work_fence_ops *ops);
> +void drm_work_fence_get(struct drm_work_fence *wfence);
> +void drm_work_fence_put(struct drm_work_fence *wfence);
> +int drm_work_fence_add_callback(struct drm_work_fence *wfence,
> + struct dma_fence *fence);
> +bool drm_work_fence_cancel(struct drm_work_fence *wfence);
> +void drm_work_fence_cancel_sync(struct drm_work_fence *wfence);
> +
> +#endif /* __DRM_WORK_FENCE_H__ */
> --
> 2.34.1
>
