On 8/28/2026 9:08 PM, Christian König wrote:
On 8/28/26 11:53, Zhu Lingshan wrote:
This commit introduces a new helper
amdgpu_lookup_queue_by_doorbell which helps
look up a user queue with the given doorbell id
in a xarray.
This function takes a kref of the user space queue.
Hello Christian
Thanks for your comments.
Well absolutely clear NAK to the whole approach.
This is the nonsense Sunil and I have worked quite hard to remove and we certainly shouldn't repeat such mistakes.
When the userq needs to be used from interrupt context we need to hold the xa_lock_irqsave() or otherwise we don't have any guarantee that the userq, userq_mgr or associated fpriv went out of scope.
Holding the spin lock by xa_lock_irqsave() can surely avoid racing with the destruction process, however, it does not apply to all scenarios, for example, you can not hold spin lock in mes_userq_reset_queue(),
because it calls either amdgpu_mes_reset_queue_mmio or amdgpu_mes_reset_queue_mmio, both of them acquire the MES mutex through amdgpu_mes_lock.
Another thing, out of the topic is, holding xa_lock does not guarantee fpriv/userq_mgr alive, for example, when drm_device->unplugged is true, all amdgpu teardown paths in amdgpu_drm_release are skipped,
and the fpriv/userq_mgr is freed, no matter whether holding the xa spin lock.
So IMHO since we have userq->kref, lets use it to maintain the lifecycle of the queues.
Grabbing references from this side would obviously result in circle dependencies.
I am not sure, we should use the lock/unlock and kref_put/get in pairs in sequence, can you name some circle dependencies or AB-BA lockings as examples?
Thanks
Lingshan
Regards,
Christian.
Signed-off-by: Zhu Lingshan <lingshan.zhu@amd.com>
---
drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c | 30 +++++++++++++++++++++++
drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h | 2 ++
2 files changed, 32 insertions(+)
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c
index 0a816b3c5ff9..e0639f844a8e 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c
@@ -609,6 +609,36 @@ struct amdgpu_usermode_queue *amdgpu_userq_get(struct amdgpu_userq_mgr *uq_mgr,
return queue;
}
+/**
+ * amdgpu_lookup_queue_by_doorbell - look up a user queue by doorbell
+ * @xa: user queue XArray indexed by doorbell
+ * @doorbell: doorbell index
+ *
+ * Return: A queue with the doorbell indexed, or NULL if no such a queue found.
+ *
+ * This function increases kref of the queue, the caller
+ * must release the reference with amdgpu_userq_put().
+ */
+struct amdgpu_usermode_queue *
+amdgpu_lookup_queue_by_doorbell(struct xarray *xa, u32 doorbell)
+{
+ struct amdgpu_usermode_queue *queue;
+ unsigned long flags;
+
+ xa_lock_irqsave(xa, flags);
+ queue = xa_load(xa, doorbell);
+ if (!queue)
+ goto out_unlock;
+
+ if (!kref_get_unless_zero(&queue->refcount))
+ queue = NULL;
+
+out_unlock:
+ xa_unlock_irqrestore(xa, flags);
+
+ return queue;
+}
+
void amdgpu_userq_put(struct amdgpu_usermode_queue *queue)
{
if (queue)
diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h
index 6412a7f7b6ef..8fc73862f64e 100644
--- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h
+++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h
@@ -151,6 +151,8 @@ struct amdgpu_db_info {
};
struct amdgpu_usermode_queue *amdgpu_userq_get(struct amdgpu_userq_mgr *uq_mgr, u32 qid);
+struct amdgpu_usermode_queue *
+amdgpu_lookup_queue_by_doorbell(struct xarray *xa, u32 doorbell);
void amdgpu_userq_put(struct amdgpu_usermode_queue *queue);
int amdgpu_userq_ioctl(struct drm_device *dev, void *data, struct drm_file *filp);