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

[Alice Ryhl <aliceryhl@google.com>]

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>