The quilt patch titled Subject: rust: maple_tree: rcu_read_lock() in destructor to silence lockdep has been removed from the -mm tree. Its filename was rust-maple_tree-rcu_read_lock-in-destructor-to-silence-lockdep.patch
This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
------------------------------------------------------ From: Alice Ryhl aliceryhl@google.com Subject: rust: maple_tree: rcu_read_lock() in destructor to silence lockdep Date: Wed, 17 Dec 2025 13:10:37 +0000
When running the Rust maple tree kunit tests with lockdep, you may trigger a warning that looks like this:
lib/maple_tree.c:780 suspicious rcu_dereference_check() usage!
other info that might help us debug this:
rcu_scheduler_active = 2, debug_locks = 1 no locks held by kunit_try_catch/344.
stack backtrace: CPU: 3 UID: 0 PID: 344 Comm: kunit_try_catch Tainted: G N 6.19.0-rc1+ #2 NONE Tainted: [N]=TEST Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014 Call Trace: <TASK> dump_stack_lvl+0x71/0x90 lockdep_rcu_suspicious+0x150/0x190 mas_start+0x104/0x150 mas_find+0x179/0x240 _RINvNtCs5QSdWC790r4_4core3ptr13drop_in_placeINtNtCs1cdwasc6FUb_6kernel10maple_tree9MapleTreeINtNtNtBL_5alloc4kbox3BoxlNtNtB1x_9allocator7KmallocEEECsgxAQYCfdR72_25doctests_kernel_generated+0xaf/0x130 rust_doctest_kernel_maple_tree_rs_0+0x600/0x6b0 ? lock_release+0xeb/0x2a0 ? kunit_try_catch_run+0x210/0x210 kunit_try_run_case+0x74/0x160 ? kunit_try_catch_run+0x210/0x210 kunit_generic_run_threadfn_adapter+0x12/0x30 kthread+0x21c/0x230 ? __do_trace_sched_kthread_stop_ret+0x40/0x40 ret_from_fork+0x16c/0x270 ? __do_trace_sched_kthread_stop_ret+0x40/0x40 ret_from_fork_asm+0x11/0x20 </TASK>
This is because the destructor of maple tree calls mas_find() without taking rcu_read_lock() or the spinlock. Doing that is actually ok in this case since the destructor has exclusive access to the entire maple tree, but it triggers a lockdep warning. To fix that, take the rcu read lock.
In the future, it's possible that memory reclaim could gain a feature where it reallocates entries in maple trees even if no user-code is touching it. If that feature is added, then this use of rcu read lock would become load-bearing, so I did not make it conditional on lockdep.
We have to repeatedly take and release rcu because the destructor of T might perform operations that sleep.
Link: https://lkml.kernel.org/r/20251217-maple-drop-rcu-v1-1-702af063573f@google.c... Fixes: da939ef4c494 ("rust: maple_tree: add MapleTree") Signed-off-by: Alice Ryhl aliceryhl@google.com Reported-by: Andreas Hindborg a.hindborg@kernel.org Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/x/topic/x/near/56421510... Reviewed-by: Gary Guo gary@garyguo.net Reviewed-by: Daniel Almeida daniel.almeida@collabora.com Cc: Andrew Ballance andrewjballance@gmail.com Cc: Bj��rn Roy Baron bjorn3_gh@protonmail.com Cc: Boqun Feng boqun.feng@gmail.com Cc: Danilo Krummrich dakr@kernel.org Cc: Liam Howlett liam.howlett@oracle.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Miguel Ojeda ojeda@kernel.org Cc: Trevor Gross tmgross@umich.edu Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
rust/kernel/maple_tree.rs | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)
--- a/rust/kernel/maple_tree.rs~rust-maple_tree-rcu_read_lock-in-destructor-to-silence-lockdep +++ a/rust/kernel/maple_tree.rs @@ -265,7 +265,16 @@ impl<T: ForeignOwnable> MapleTree<T> { loop { // This uses the raw accessor because we're destroying pointers without removing them // from the maple tree, which is only valid because this is the destructor. - let ptr = ma_state.mas_find_raw(usize::MAX); + // + // Take the rcu lock because mas_find_raw() requires that you hold either the spinlock + // or the rcu read lock. This is only really required if memory reclaim might + // reallocate entries in the tree, as we otherwise have exclusive access. That feature + // doesn't exist yet, so for now, taking the rcu lock only serves the purpose of + // silencing lockdep. + let ptr = { + let _rcu = kernel::sync::rcu::Guard::new(); + ma_state.mas_find_raw(usize::MAX) + }; if ptr.is_null() { break; } _
Patches currently in -mm which might be from aliceryhl@google.com are