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

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Message-ID: <b100a3c4-e5c3-41da-8c02-3a4986b49eec@linux.intel.com>
Date: Thu, 23 May 2024 21:44:33 +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 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?

If so,

	if (cmpxchg64(*ptr, 0, new) != 0)

is not equivalent to

	if (!try_cmpxchg64(*ptr, &tmp, new))

Best regards,
baolu