[PATCH] rust_binder: remove spin_lock() in rust_shrink_free_page()

[PATCH] rust_binder: remove spin_lock() in rust_shrink_free_page()

[Alice Ryhl]

When forward-porting Rust Binder to 6.18, I neglected to take commit
fb56fdf8b9a2 ("mm/list_lru: split the lock to per-cgroup scope") into
account, and apparently I did not end up running the shrinker callback
when I sanity tested the driver before submission. This leads to crashes
like the following:

	============================================
	WARNING: possible recursive locking detected
	6.18.0-mainline-maybe-dirty #1 Tainted: G          IO
	--------------------------------------------
	kswapd0/68 is trying to acquire lock:
	ffff956000fa18b0 (&l->lock){+.+.}-{2:2}, at: lock_list_lru_of_memcg+0x128/0x230

	but task is already holding lock:
	ffff956000fa18b0 (&l->lock){+.+.}-{2:2}, at: rust_helper_spin_lock+0xd/0x20

	other info that might help us debug this:
	 Possible unsafe locking scenario:

	       CPU0
	       ----
	  lock(&l->lock);
	  lock(&l->lock);

	 *** DEADLOCK ***

	 May be due to missing lock nesting notation

	3 locks held by kswapd0/68:
	 #0: ffffffff90d2e260 (fs_reclaim){+.+.}-{0:0}, at: kswapd+0x597/0x1160
	 #1: ffff956000fa18b0 (&l->lock){+.+.}-{2:2}, at: rust_helper_spin_lock+0xd/0x20
	 #2: ffffffff90cf3680 (rcu_read_lock){....}-{1:2}, at: lock_list_lru_of_memcg+0x2d/0x230

To fix this, remove the spin_lock() call from rust_shrink_free_page().

Cc: stable@vger.kernel.org
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 drivers/android/binder/page_range.rs | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/android/binder/page_range.rs b/drivers/android/binder/page_range.rs
index 9379038f61f513c51ebed6c7e7b6fde32e5b8d06..fdd97112ef5c8b2341e498dc3567b659f05e3fd7 100644
--- a/drivers/android/binder/page_range.rs
+++ b/drivers/android/binder/page_range.rs
@@ -727,8 +727,5 @@ fn drop(self: Pin<&mut Self>) {
     drop(mm);
     drop(page);
 
-    // SAFETY: We just unlocked the lru lock, but it should be locked when we return.
-    unsafe { bindings::spin_lock(&raw mut (*lru).lock) };
-
     LRU_REMOVED_ENTRY
 }

---
base-commit: 82d12088c297fa1cef670e1718b3d24f414c23f7
change-id: 20251202-binder-shrink-unspin-1ee9872cc2ed

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>