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

[Baolu Lu <baolu.lu@linux.intel.com>]

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