Message-ID: <b4de4547-84ed-4d3d-9102-ccb878f6016b@intel.com>
Date: Mon, 16 Jun 2025 08:22:39 -0700
From: Dave Jiang <dave.jiang@...el.com>
To: Shuai Xue <xueshuai@...ux.alibaba.com>,
 Vinicius Costa Gomes <vinicius.gomes@...el.com>, fenghuay@...dia.com,
 vkoul@...nel.org
Cc: dmaengine@...r.kernel.org, colin.i.king@...il.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] dmaengine: idxd: Fix race condition between WQ
 enable and reset paths



On 6/15/25 6:54 PM, Shuai Xue wrote:
> 
> 
> 在 2025/6/6 22:32, Dave Jiang 写道:
>>
>>
>> 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@...ux.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.
> 
> Hi, Dave,
> 
> I split patch 2 out.
> 
> As for this race issue between WQ enable and reset paths, do you have any plan
> or idea?

Need a lock to protect the paths. Maybe a patch to use the existing wq lock for now and somebody can clean up the locking after that? I don't work on this driver anymore. So either you can propose a locking cleanup or if Vinicius has time maybe he can take a look. 

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

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