linux-kernel - Re: [PATCH] radix-tree: Fix private list warnings

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Date:   Sat, 14 Jan 2017 19:57:51 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Matthew Wilcox <mawilcox@...rosoft.com>
Cc:     Matthew Wilcox <mawilcox@...uxonhyperv.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] radix-tree: Fix private list warnings

On Sat, Jan 14, 2017 at 09:31:51PM +0000, Matthew Wilcox wrote:
> From: Johannes Weiner [mailto:hannes@...xchg.org]
> > On Thu, Jan 12, 2017 at 05:28:23PM -0800, Matthew Wilcox wrote:
> > > The newly introduced warning in radix_tree_free_nodes() was testing the
> > > wrong variable; it should have been 'old' instead of 'node'.  Rather
> > > than replace that one instance, I noticed that we can simply put the
> > > WARN_ON_ONCE in radix_tree_node_free() and it will be just as effective.
> > >
> > > Fixes: ea07b862ac8e ("mm: workingset: fix use-after-free in shadow node
> > shrinker")
> > > Signed-off-by: Matthew Wilcox <mawilcox@...rosoft.com>
> > 
> > Thanks for the fix in radix_tree_free_nodes(). But I intentionally
> > spread out the warnings to retain the line numbers. The inlining in
> > that code will obscure which tree operation ends up triggering.
> 
> The backtrace should make that fairly obvious though; even if
> radix_tree_node_free() gets inlined, the caller of
> radix_tree_free_nodes() probably doesn't also call
> radix_tree_shrink() or delete_node().  You're really looking for the
> caller in the backtrace that's outside of radix-tree.c.

Distinguishing between the radix_tree_shrink() and the delete_node()
sites was essential to find the bug whose fix added these warnings.
The former gets inlined into the latter. Not impossible to figure out
which one triggered from a full dump, but certainly less robust.

> By the way, if you'd run the radix tree test-suite, the WARN_ON_ONCE
> would have triggered (that's how I spotted the bug).  The test-suite
> is getting pretty useful these days; would you mind running it in
> future?

Thanks, I'll keep that in mind for future radix tree changes.