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

[Baolu Lu]

On 5/23/24 9:57 PM, Uros Bizjak wrote:
> On Thu, May 23, 2024 at 3:44 PM Baolu Lu<baolu.lu@xxxxxxxxxxxxxxx>  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