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

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Date:   Sat, 14 Jan 2017 21:31:51 +0000
From:   Matthew Wilcox <mawilcox@...rosoft.com>
To:     Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <mawilcox@...uxonhyperv.com>
CC:     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

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.

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?