Re: [PATCH 01/10] drm/amdgpu: introduce amdgpu_lookup_queue_by_doorbell

["Zhu, Lingshan" <lingshan.zhu@amd.com>]

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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);

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