Re: [PATCH v1 1/2] dmaengine: idxd: Fix race condition between WQ enable and reset paths

[Dave Jiang <dave.jiang@intel.com>]

On 6/5/25 12:40 AM, Shuai Xue wrote:
> 
> 
> 在 2025/6/4 22:19, Dave Jiang 写道:
>>
>>
>> On 6/4/25 1:55 AM, Shuai Xue wrote:
>>>
>>>
>>> 在 2025/6/3 22:32, Dave Jiang 写道:
>>>>
>>>>
>>>> On 5/27/25 7:21 PM, Vinicius Costa Gomes wrote:
>>>>> Shuai Xue <xueshuai@linux.alibaba.com> writes:
>>>>>
>>>>>> 在 2025/5/23 22:54, Dave Jiang 写道:
>>>>>>>
>>>>>>>
>>>>>>> On 5/22/25 10:20 PM, Shuai Xue wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> 在 2025/5/22 22:55, Dave Jiang 写道:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 5/21/25 11:33 PM, Shuai Xue wrote:
>>>>>>>>>> A device reset command disables all WQs in hardware. If issued while a WQ
>>>>>>>>>> is being enabled, it can cause a mismatch between the software and hardware
>>>>>>>>>> states.
>>>>>>>>>>
>>>>>>>>>> When a hardware error occurs, the IDXD driver calls idxd_device_reset() to
>>>>>>>>>> send a reset command and clear the state (wq->state) of all WQs. It then
>>>>>>>>>> uses wq_enable_map (a bitmask tracking enabled WQs) to re-enable them and
>>>>>>>>>> ensure consistency between the software and hardware states.
>>>>>>>>>>
>>>>>>>>>> However, a race condition exists between the WQ enable path and the
>>>>>>>>>> reset/recovery path. For example:
>>>>>>>>>>
>>>>>>>>>> A: WQ enable path                   B: Reset and recovery path
>>>>>>>>>> ------------------                 ------------------------
>>>>>>>>>> a1. issue IDXD_CMD_ENABLE_WQ
>>>>>>>>>>                                        b1. issue IDXD_CMD_RESET_DEVICE
>>>>>>>>>>                                        b2. clear wq->state
>>>>>>>>>>                                        b3. check wq_enable_map bit, not set
>>>>>>>>>> a2. set wq->state = IDXD_WQ_ENABLED
>>>>>>>>>> a3. set wq_enable_map
>>>>>>>>>>
>>>>>>>>>> In this case, b1 issues a reset command that disables all WQs in hardware.
>>>>>>>>>> Since b3 checks wq_enable_map before a2, it doesn't re-enable the WQ,
>>>>>>>>>> leading to an inconsistency between wq->state (software) and the actual
>>>>>>>>>> hardware state (IDXD_WQ_DISABLED).
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Would it lessen the complication to just have wq enable path grab the device lock before proceeding?
>>>>>>>>>
>>>>>>>>> DJ
>>>>>>>>
>>>>>>>> Yep, how about add a spin lock to enable wq and reset device path.
>>>>>>>>
>>>>>>>> diff --git a/drivers/dma/idxd/device.c b/drivers/dma/idxd/device.c
>>>>>>>> index 38633ec5b60e..c0dc904b2a94 100644
>>>>>>>> --- a/drivers/dma/idxd/device.c
>>>>>>>> +++ b/drivers/dma/idxd/device.c
>>>>>>>> @@ -203,6 +203,29 @@ int idxd_wq_enable(struct idxd_wq *wq)
>>>>>>>>     }
>>>>>>>>     EXPORT_SYMBOL_GPL(idxd_wq_enable);
>>>>>>>>     +/*
>>>>>>>> + * This function enables a WQ in hareware and updates the driver maintained
>>>>>>>> + * wq->state to IDXD_WQ_ENABLED. It should be called with the dev_lock held
>>>>>>>> + * to prevent race conditions with IDXD_CMD_RESET_DEVICE, which could
>>>>>>>> + * otherwise disable the WQ without the driver's state being properly
>>>>>>>> + * updated.
>>>>>>>> + *
>>>>>>>> + * For IDXD_CMD_DISABLE_DEVICE, this function is safe because it is only
>>>>>>>> + * called after the WQ has been explicitly disabled, so no concurrency
>>>>>>>> + * issues arise.
>>>>>>>> + */
>>>>>>>> +int idxd_wq_enable_locked(struct idxd_wq *wq)
>>>>>>>> +{
>>>>>>>> +       struct idxd_device *idxd = wq->idxd;
>>>>>>>> +       int ret;
>>>>>>>> +
>>>>>>>> +       spin_lock(&idxd->dev_lock);
>>>>>>>
>>>>>>> Let's start using the new cleanup macro going forward:
>>>>>>> guard(spinlock)(&idxd->dev_lock);
>>>>>>>
>>>>>>> On a side note, there's been a cleanup on my mind WRT this driver's locking. I think we can replace idxd->dev_lock with idxd_confdev(idxd) device lock. You can end up just do:
>>>>>>> guard(device)(idxd_confdev(idxd));
>>>>>>
>>>>>> Then we need to replace the lock from spinlock to mutex lock?
>>>>>
>>>>> We still need a (spin) lock that we could hold in interrupt contexts.
>>>>>
>>>>>>
>>>>>>>
>>>>>>> And also drop the wq->wq_lock and replace with wq_confdev(wq) device lock:
>>>>>>> guard(device)(wq_confdev(wq));
>>>>>>>
>>>>>>> If you are up for it that is.
>>>>>>
>>>>>> We creates a hierarchy: pdev -> idxd device -> wq device.
>>>>>> idxd_confdev(idxd) is the parent of wq_confdev(wq) because:
>>>>>>
>>>>>>        (wq_confdev(wq))->parent = idxd_confdev(idxd);
>>>>>>
>>>>>> Is it safe to grap lock of idxd_confdev(idxd) under hold
>>>>>> lock of wq_confdev(wq)?
>>>>>>
>>>>>> We have mounts of code use spinlock of idxd->dev_lock under
>>>>>> hold of wq->wq_lock.
>>>>>>
>>>>>
>>>>> I agree with Dave that the locking could be simplified, but I don't
>>>>> think that we should hold this series because of that. That
>>>>> simplification can be done later.
>>>>
>>>> I agree. Just passing musing on the current code.
>>>
>>> Got it, do I need to send a separate patch for Patch 2?
>>
>> Not sure what you mean. Do you mean if you need to send patch 2 again?
> 
> Yep, the locking issue is more complicate and can be done later.
> (I could split patch 2 from this patch set if you prefer a new patch.)

Yes split it out and send it ahead if it can go independently.

> 
>>>
>>> Thanks.
>>> Shuai
> 
>