linux-kernel - Re: [patch] mm: fix lockless pagecache reordering bug (was Re:

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 05 Jan 2009 22:04:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Klotz <peter.klotz@....at>, stable@...nel.org,
	Linux Memory Management List <linux-mm@...ck.org>,
	Christoph Hellwig <hch@...radead.org>,
	Roman Kononov <kernel@...onov.ftml.net>,
	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [patch] mm: fix lockless pagecache reordering bug (was Re:

On Mon, 2009-01-05 at 12:12 -0800, Paul E. McKenney wrote:
> On Mon, Jan 05, 2009 at 10:44:27AM -0800, Linus Torvalds wrote:
> > On Mon, 5 Jan 2009, Nick Piggin wrote:
> > > On Mon, Jan 05, 2009 at 09:30:55AM -0800, Linus Torvalds wrote:
> > > Putting an rcu_dereference there might work, but I think it misses a 
> > > subtlety of this code.
> > 
> > No, _you_ miss the subtlety of something that can change under you.
> > 
> > Look at radix_tree_deref_slot(), and realize that without the 
> > rcu_dereference(), the compiler would actually be allowed to think that it 
> > can re-load anything from *pslot several times. So without my one-liner 
> > patch, the compiler can actually do this:
> > 
> > 	register = load_from_memory(pslot)
> > 	if (radix_tree_is_indirect_ptr(register))
> > 		goto fail:
> > 	return load_from_memory(pslot);
> > 
> >    fail:
> > 	return RADIX_TREE_RETRY;
> 
> My guess is that Nick believes that the value in *pslot cannot change
> in such as way as to cause radix_tree_is_indirect_ptr()'s return value
> to change within a given RCU grace period, and that Linus disagrees.

Nick's belief would indeed be true IFF all modifying ops including all
uses of radix_tree_replace_slot() are serialized wrt. each other.

However, since radix_tree_deref_slot() is the counterpart of
radix_tree_replace_slot(), one would indeed expect rcu_dereference()
therein, much like Linus suggests.

While what Nick says is true, the lifetime management of the data
objects is arranged externally from the radix tree -- I still think we
need the rcu_dereference() even for that argument, as we want to support
RCU lifetime management as well.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/