Re: [PATCH] rust: maple_tree: rcu_read_lock() in destructor to silence lockdep

[Daniel Almeida <daniel.almeida@collabora.com>]

> On 17 Dec 2025, at 10:10, Alice Ryhl <aliceryhl@google.com> wrote:
> 
> 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.
> 
> Reported-by: Andreas Hindborg <a.hindborg@kernel.org>
> Closes: https://rust-for-linux.zulipchat.com/#narrow/channel/x/topic/x/near/564215108
> Fixes: da939ef4c494 ("rust: maple_tree: add MapleTree")
> Cc: stable@vger.kernel.org
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> Intended for the same tree as any other maple tree patch. (I believe
> that's Andrew Morton's tree.)
> ---
> rust/kernel/maple_tree.rs | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/maple_tree.rs b/rust/kernel/maple_tree.rs
> index e72eec56bf5772ada09239f47748cd649212d8b0..265d6396a78a17886c8b5a3ebe7ba39ccc354add 100644
> --- a/rust/kernel/maple_tree.rs
> +++ b/rust/kernel/maple_tree.rs
> @@ -265,7 +265,16 @@ unsafe fn free_all_entries(self: Pin<&mut Self>) {
>         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;
>             }
> 
> ---
> base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
> change-id: 20251217-maple-drop-rcu-dfe72fb5f49e
> 
> Best regards,
> -- 
> Alice Ryhl <aliceryhl@google.com>
> 
> 

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>