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Date: Thu, 4 Feb 2016 11:44:02 +0300
From: Konstantin Khlebnikov <koct9i@...il.com>
To: Matthew Wilcox <matthew.r.wilcox@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>,
Ohad Ben-Cohen <ohad@...ery.com>,
Matthew Wilcox <willy@...ux.intel.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
Stable <stable@...r.kernel.org>
Subject: Re: [PATCH 1/5] radix-tree: Fix race in gang lookup
On Thu, Feb 4, 2016 at 12:37 AM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
> On Thu, Jan 28, 2016 at 12:17 AM, Matthew Wilcox
> <matthew.r.wilcox@...el.com> wrote:
>> From: Matthew Wilcox <willy@...ux.intel.com>
>>
>> If the indirect_ptr bit is set on a slot, that indicates we need to
>> redo the lookup. Introduce a new function radix_tree_iter_retry()
>> which forces the loop to retry the lookup by setting 'slot' to NULL and
>> turning the iterator back to point at the problematic entry.
>>
>> This is a pretty rare problem to hit at the moment; the lookup has to
>> race with a grow of the radix tree from a height of 0. The consequences
>> of hitting this race are that gang lookup could return a pointer to a
>> radix_tree_node instead of a pointer to whatever the user had inserted
>> in the tree.
>>
>> Fixes: cebbd29e1c2f ("radix-tree: rewrite gang lookup using iterator")
>> Signed-off-by: Matthew Wilcox <willy@...ux.intel.com>
>> Cc: stable@...r.kernel.org
>> ---
>> include/linux/radix-tree.h | 16 ++++++++++++++++
>> lib/radix-tree.c | 12 ++++++++++--
>> 2 files changed, 26 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
>> index f9a3da5bf892..db0ed595749b 100644
>> --- a/include/linux/radix-tree.h
>> +++ b/include/linux/radix-tree.h
>> @@ -387,6 +387,22 @@ void **radix_tree_next_chunk(struct radix_tree_root *root,
>> struct radix_tree_iter *iter, unsigned flags);
>>
>> /**
>> + * radix_tree_iter_retry - retry this chunk of the iteration
>> + * @iter: iterator state
>> + *
>> + * If we iterate over a tree protected only by the RCU lock, a race
>> + * against deletion or creation may result in seeing a slot for which
>> + * radix_tree_deref_retry() returns true. If so, call this function
>> + * and continue the iteration.
>> + */
>> +static inline __must_check
>> +void **radix_tree_iter_retry(struct radix_tree_iter *iter)
>> +{
>> + iter->next_index = iter->index;
>> + return NULL;
>> +}
>> +
>> +/**
>> * radix_tree_chunk_size - get current chunk size
>> *
>> * @iter: pointer to radix tree iterator
>> diff --git a/lib/radix-tree.c b/lib/radix-tree.c
>> index a25f635dcc56..65422ac17114 100644
>> --- a/lib/radix-tree.c
>> +++ b/lib/radix-tree.c
>> @@ -1105,9 +1105,13 @@ radix_tree_gang_lookup(struct radix_tree_root *root, void **results,
>> return 0;
>>
>> radix_tree_for_each_slot(slot, root, &iter, first_index) {
>> - results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
>> + results[ret] = rcu_dereference_raw(*slot);
>> if (!results[ret])
>> continue;
>> + if (radix_tree_is_indirect_ptr(results[ret])) {
>> + slot = radix_tree_iter_retry(&iter);
>> + continue;
>> + }
>> if (++ret == max_items)
>> break;
>> }
>
> Looks like your fix doesn't work.
>
> After radix_tree_iter_retry: radix_tree_for_each_slot will call
> radix_tree_next_slot which isn't safe to call for NULL slot.
>
> #define radix_tree_for_each_slot(slot, root, iter, start) \
> for (slot = radix_tree_iter_init(iter, start) ; \
> slot || (slot = radix_tree_next_chunk(root, iter, 0)) ; \
> slot = radix_tree_next_slot(slot, iter, 0))
>
> tagged iterator works becase restart happens only at root - tags
> filled with single bit.
>
> quick (untested) fix for that
>
> --- a/include/linux/radix-tree.h
> +++ b/include/linux/radix-tree.h
> @@ -457,9 +457,9 @@ radix_tree_next_slot(void **slot, struct
> radix_tree_iter *iter, unsigned flags)
> return slot + offset + 1;
> }
> } else {
> - unsigned size = radix_tree_chunk_size(iter) - 1;
> + int size = radix_tree_chunk_size(iter) - 1;
>
> - while (size--) {
> + while (size-- > 0) {
> slot++;
> iter->index++;
> if (likely(*slot))
>
>
Yep. Kernel crashes. Test in attachment.
fix: https://lkml.kernel.org/r/145457528789.31321.4441662473067711123.stgit@zurg
>> @@ -1184,9 +1188,13 @@ radix_tree_gang_lookup_tag(struct radix_tree_root *root, void **results,
>> return 0;
>>
>> radix_tree_for_each_tagged(slot, root, &iter, first_index, tag) {
>> - results[ret] = indirect_to_ptr(rcu_dereference_raw(*slot));
>> + results[ret] = rcu_dereference_raw(*slot);
>> if (!results[ret])
>> continue;
>> + if (radix_tree_is_indirect_ptr(results[ret])) {
>> + slot = radix_tree_iter_retry(&iter);
>> + continue;
>> + }
>> if (++ret == max_items)
>> break;
>> }
>> --
>> 2.7.0.rc3
>>
>> --
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