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Message-ID: <a6a8f04b-0dd8-462b-8bad-8b7c2dc0d736@linux.intel.com>
Date: Fri, 24 May 2024 09:08:04 +0800
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Cc: baolu.lu@linux.intel.com, iommu@lists.linux.dev,
 linux-kernel@vger.kernel.org, David Woodhouse <dwmw2@infradead.org>,
 Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
 Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH 2/3] iommu/vt-d: Use try_cmpxchg64() in
 intel_pasid_get_entry()
To: Uros Bizjak <ubizjak@gmail.com>
References: <20240522082729.971123-1-ubizjak@gmail.com>
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 <c1892ba9-4eae-40fc-b3ab-73d0f82a74ea@linux.intel.com>
 <CAFULd4Z=YkV1Hbs4DikPBwO-6rg8tfDLGeacSCnfbC02E5y+Cg@mail.gmail.com>
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 <CAFULd4aXBAxPS7hXJ_RKMzZu60yTr7gK1m3K8z0yq1mjYn3dyA@mail.gmail.com>
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From: Baolu Lu <baolu.lu@linux.intel.com>
In-Reply-To: <CAFULd4aXBAxPS7hXJ_RKMzZu60yTr7gK1m3K8z0yq1mjYn3dyA@mail.gmail.com>
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On 5/23/24 9:57 PM, Uros Bizjak wrote:
> On Thu, May 23, 2024 at 3:44 PM Baolu Lu<baolu.lu@linux.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