Re: [PATCH] maple_tree: Annotate lockless pivot reads for KCSAN
From: Hui Zhu
Date: Wed Sep 02 2026 - 07:52:53 EST
On 26/08/27 05:35AM, Hui Zhu wrote:
Isn't your change already using the data_racy annotation on more thanOn 26/08/26 09:41AM, Andrew Morton wrote:Hi Liam,
On Wed, 26 Aug 2026 15:44:30 +0800 "Hui Zhu" <hui.zhu@xxxxxxxxx> wrote:Yes, this is literally every read of numerous pivots on any reader that
In RCU mode, replaced maple nodes are marked dead and freed via RCU
after the new node has been published. Arming the RCU free writes
node->rcu.next and node->rcu.func, which share storage with
pivot[0] and pivot[1] (see struct maple_node), while lockless
readers may still walk the dead node. These stores therefore race
with the pivot loads performed by the walkers.
This is harmless: the writer marks the node dead with an smp_wmb()
before arming the rcu_head, and the walkers re-check ma_dead_node()
after reading the node and restart the walk when the node is dead,
so any pivot read that raced with the rcu_head stores is discarded.
KCSAN cannot see this protocol and reports the plain accesses, so
annotate the lockless pivot reads with data_race() through a new
ma_pivot_rcu() helper.
Found by fuzzing on a 6.6 kernel; the race still exists on
mainline. No functional change intended.
Thanks. AI review suggests that the patch is correct, but incomplete?
https://sashiko.dev/#/patchset/20260826074430.1139325-1-hui.zhu@xxxxxxxxx
would need something like this. Most code is shared with the writer
side, so we'll have data_race() annotation where it is not needed there.
I don't like the name of the function and I don't agree that it is worth
doing, especially half of it. If you notice ma_ functions take a maple
node as the first argument, but your new function takes an array pointer
in the node.
The from address does not agree with the sign-off on the patch.
I also don't think a benign race needs a Fixes tag?
It might be worth looking at other ways to calm kcsans down such as the
type qualifier __data_racy, or maybe the makefile option.
__data_racy is defined as volatile for KCSAN kernels, so the qualifier
has to propagate to every access site: ma_pivots() would return a
volatile pointer, and all the "unsigned long *pivots" locals and helper
parameters in maple_tree.c (about 25 sites) would need the qualifier
too.
It also marks the whole pivot array as racy, while only pivot[0] and
pivot[1] actually overlap the rcu_head.
pivot 0 and 1? For instance, mas->offset is often passed in and that
is likely not 0 or 1.
You're right, and I looked into annotating only pivot[0] and pivot[1],
which are the only pivots that overlap the rcu_head of a dead node.
The trouble is that the index being read is only known at run time
(pivots[offset], pivots[mas->offset], ...), so the only way to wrap
just the pivot[0]/pivot[1] reads in data_race() is a run-time check
before each read.
For example, mtree_range_walk() would turn from:
if (pivots[offset] >= mas->index) {
into:
unsigned long pivot;
if (offset < 2)
pivot = data_race(pivots[offset]);
else
pivot = pivots[offset];
if (pivot >= mas->index) {
These walks are the lockless hot path (every RCU lookup, including
the page-fault path), so this adds a branch to every pivot read.
data_race() compiles to nothing without CONFIG_KCSAN, so the annotation
itself is free in production kernels, but the branch would cost on every
build.
I don't want to add branches to this core code just to narrow the
annotation, and I'd also rather not widen the suppression to pivots that
never race.
Given that, I'm inclined to drop this patch.
Instead I could add a comment where the rcu_head is armed,
noting that the pivot[0]/pivot[1] reads on the lockless paths are
expected to race and that the resulting KCSAN reports are benign,
so anyone hitting the same report later knows to ignore it.
Does that sound reasonable, or do you have another suggestion?
Best,
Hui
Fair point about the code churn, though. I don't need every function
being changed to accept a volatile.
The Makefile option (KCSAN_SANITIZE_maple_tree.o := n) is a one-liner,KCSAN has never reported anything real to me and that's why I suggested
but it disables KCSAN for the entire file, so any real data race
introduced in maple_tree.c later would go unnoticed.
turning it off. After speaking with Paul McKenney on the matter I have
been convinced to not disable it.
I still don't really see a whole lot of value in annotating the code to
say a particular race is not an issue - we will have mostly disabled the
tool by annotation in the end. And these are most likely going to be
the areas where we have issues if any arise - ie, some special arch
corner case that comes up that didn't exist or wasn't in mind during
annotation.
Do you think one of these two is still the better choice, or should II guess respin it and see how it looks with your current path.
keep the current approach and fix the patch according to your review
comments?
Thanks,
Liam