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

[Linus Torvalds <torvalds@osdl.org>]

On Wed, 26 May 2004, Benjamin Herrenschmidt wrote:
> 
> I think we are using ptep_establish for more than just setting those
> 2 bits (like for setting up the new PTE in break_cow and such while
> the current implementationL/definition is more like just setting
> those bits and nothing else....

You're right. We do use it on the do_wp_page() path, and there we actually 
use a whole new page in the "break_cow()" case. That case is in fact 
fundamentally different from the other ones.

So we should probably break up the "ptep_establish()" into its two pieces,
since the callers don't actually want to do the same thing. One really
wants to do a "clear old one, set a totally new one", and the two other
places want to actually update just the dirty and accessed bits.

In fact, the only non-generic user of "ptep_establish()" (s390) didn't 
want to use the generic version exactly because of this very conceptual 
bug. It uses "ptep_clear_flush()" for the replacement case, which actually 
makes sense.

So does it work if you do this appended patch first? This is a real 
cleanup, and I think it will allow us to get rid of the s390-specific code 
in ptep_establish(). Along with hopefully fixing your problem too.

After this, we should be able to have a BUG() in "set_pte()" if the entry 
wasn't clear before (assuming the arch doesn't use set_pte() for the dirty 
updates etc).

		Linus

---
===== mm/memory.c 1.177 vs edited =====
--- 1.177/mm/memory.c	Tue May 25 12:37:09 2004
+++ edited/mm/memory.c	Tue May 25 21:04:49 2004
@@ -1004,7 +1004,10 @@
 	flush_cache_page(vma, address);
 	entry = maybe_mkwrite(pte_mkdirty(mk_pte(new_page, vma->vm_page_prot)),
 			      vma);
-	ptep_establish(vma, address, page_table, entry, 1);
+
+	/* Get rid of the old entry, replace with new */
+	ptep_clear_flush(vma, address, page_table);
+	set_pte(page_table, entry);
 	update_mmu_cache(vma, address, entry);
 }
 
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