Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping

On Mon, Dec 13, 2021 at 08:37:04AM +0100, David Hildenbrand wrote:

On 13.12.21 08:27, Jianyong Wu wrote:

quoted

quoted

quoted

@@ -359,6 +365,7 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
 	} while (pudp++, addr = next, addr != end);

 	pud_clear_fixmap();
+	spin_unlock(&fixmap_lock);
 }

 static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,

As the race could only happen with memory hotplug being enabled, could
not we wrap this around with CONFIG_MEMORY_HOTPLUG, just to narrow
its scope possibly speed up other non-hotplug cases ?

Assuming you can quantify the speed-up...

quoted

I think it's better.

We better avoid using ifdef if not really necessary, it just uglifies
the code. We could add

if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))
	...

But should we really try to micto-optimize this code and make it harder
to read for the purpose of an unproven performance gain? (if there is no
contention, we'll most probably not even recognize that we're taking a lock)

I agree, I don't see the point of this optimisation.

-- 
Catalin

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