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

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Message-ID: <97e52eb0-d531-4464-bbb7-1dffa5d8d74e@linux.alibaba.com>
Date: Thu, 5 Jun 2025 15:40:15 +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 2025/6/4 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.)

>>
>> Thanks.
>> Shuai