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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.com 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/564215108 Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com> Cc: Andrew Ballance <andrewjballance@gmail.com> Cc: Björn 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>