linux-kernel - Re: [PATCH 2/3] iommu/vt-d: Use try_cmpxchg64() in intel_pasid_get

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Date: Fri, 24 May 2024 09:08:04 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: baolu.lu@...ux.intel.com, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org, David Woodhouse <dwmw2@...radead.org>,
 Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
 Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 2/3] iommu/vt-d: Use try_cmpxchg64() in
 intel_pasid_get_entry()

On 5/23/24 9:57 PM, Uros Bizjak wrote:
> On Thu, May 23, 2024 at 3:44 PM Baolu Lu<baolu.lu@...ux.intel.com>  wrote:
>> On 2024/5/23 21:34, Uros Bizjak wrote:
>>>>> +             if (!try_cmpxchg64(&dir[dir_index].val, &tmp,
>>>>> +                                (u64)virt_to_phys(entries) | PASID_PTE_PRESENT)) {
>>>> Above change will cause a dead loop during boot. It should be
>>> No, it is correct as written:
>>>
>>> if (cmpxchg64(*ptr, 0, new))
>>>
>>> can be written as:
>>>
>>> if (cmpxchg64(*ptr, 0, new) != 0)
>>>
>>> this is equivalent to:
>>>
>>> tmp = 0ULL;
>>> if (!try_cmpxchg64(*ptr, &tmp, new))
>> The return value of both cmpxchg64() and try_cmpxchg64() is the old
>> value that was loaded from the memory location, right?
> Actually, try_cmpxchg() returns true if successful and false if it fails.

Oh! I misunderstood this.

> 
>              tmp = 0ULL;
>              if (!try_cmpxchg64(*ptr, &tmp, new))
> 
> The logic in the above snippet can be interpreted as:
> 
> if we fail to compare *ptr with 0, then:
> 
>              iommu_free_page(entries);
>              goto retry;
> 
> as intended in the original code.

Okay, it's fine.

Best regards,
baolu