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

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Date: Thu, 23 May 2024 15:57:40 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: 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 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.

            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.

Thanks,
Uros.