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

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Message-ID: <22b3a299-b148-46ec-804e-2f6cbb3d5de1@linux.alibaba.com>
Date: Wed, 4 Jun 2025 16:55:25 +0800
From: Shuai Xue <xueshuai@...ux.alibaba.com>
To: Dave Jiang <dave.jiang@...el.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



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?

Thanks.
Shuai