Re: [PATCH] ppc64: Fix possible race with set_pte on a present PTE

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> and similarly on most other architectures it should be quite easy to do 
> the equivalent. You can always do it with a simple compare-and-exchange 
> loop, something any SMP-capable architecture should have.
> 
> Of course, arguably we can actually optimize this by "knowing" that it is
> safe to set the dirty bit, so then we don't even need an atomic operation,
> we just need one atomic write.  So we only actually need the atomic op for 
> the accessed bit case, and if we make the write-case be totally separate..

Looks good ! That gives us a guarantee that set_pte is never ever called
on a present PTE (thus letting set_pte be non-atomic) and we can safely
BUG_ON(pte_present(*ptep)) in it, right ?
 
Note that having different set_dirty and set_accessed may be useful for
some archs, thouh I agree a single atomic operation is enough on ppc
too, I also want to make sure nobody ever gets the idea of using that
for anything but those 2 bits... Well, that's a matter of taste, go for
what you prefer.

ppc64 version of this would look like

static inline unsigned long ptep_set_bits(pte_t *p, unsigned long set)
{
	unsigned long old, tmp;

	__asm__ __volatile__(
	"1:	ldarx	%0,0,%3\n\
	or	%1,%0,%4 \n\
	stdcx.	%1,0,%3 \n\
	bne-	1b"
	: "=&r" (old), "=&r" (tmp), "=m" (*p)
	: "r" (p), "r" (clr), "m" (*p)
	: "cc" );
	return old;
}

ppc32 would be:

#define ptep_set_bits(p, bits) pte_update(p, 0, bits)

> Anybody willing to write up a patch for a few architectures? Is there any 
> architecture out there that would have a problem with this?
> 
> 		Linus
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-- 
Benjamin Herrenschmidt <benh@kernel.crashing.org>