btrfs: fix subpage deadlock in try_release_subpage_extent_buffer()

loemraw · kdave · commit ad580dfa388f · 2025-08-07T17:07:15.000+02:00
There is a potential deadlock that can happen in try_release_subpage_extent_buffer() because the irq-safe xarray spin lock fs_info->buffer_tree is being acquired before the irq-unsafe eb->refs_lock. This leads to the potential race: // T1 (random eb->refs user) // T2 (release folio) spin_lock(&eb->refs_lock); // interrupt end_bbio_meta_write() btrfs_meta_folio_clear_writeback() btree_release_folio() folio_test_writeback() //false try_release_extent_buffer() try_release_subpage_extent_buffer() xa_lock_irq(&fs_info->buffer_tree) spin_lock(&eb->refs_lock); // blocked; held by T1 buffer_tree_clear_mark() xas_lock_irqsave() // blocked; held by T2 I believe that the spin lock can safely be replaced by an rcu_read_lock. The xa_for_each loop does not need the spin lock as it's already internally protected by the rcu_read_lock. The extent buffer is also protected by the rcu_read_lock so it won't be freed before we take the eb->refs_lock and check the ref count. The rcu_read_lock is taken and released every iteration, just like the spin lock, which means we're not protected against concurrent insertions into the xarray. This is fine because we rely on folio->private to detect if there are any ebs remaining in the folio. There is already some precedent for this with find_extent_buffer_nolock, which loads an extent buffer from the xarray with only rcu_read_lock. lockdep warning: ===================================================== WARNING: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected 6.16.0-0_fbk701_debug_rc0_123_g4c06e63b9203 #1 Tainted: G E N ----------------------------------------------------- kswapd0/66 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire: ffff000011ffd600 (&eb->refs_lock){+.+.}-{3:3}, at: try_release_extent_buffer+0x18c/0x560 and this task is already holding: ffff0000c1d91b88 (&buffer_xa_class){-.-.}-{3:3}, at: try_release_extent_buffer+0x13c/0x560 which would create a new lock dependency: (&buffer_xa_class){-.-.}-{3:3} -> (&eb->refs_lock){+.+.}-{3:3} but this new dependency connects a HARDIRQ-irq-safe lock: (&buffer_xa_class){-.-.}-{3:3} ... which became HARDIRQ-irq-safe at: lock_acquire+0x178/0x358 _raw_spin_lock_irqsave+0x60/0x88 buffer_tree_clear_mark+0xc4/0x160 end_bbio_meta_write+0x238/0x398 btrfs_bio_end_io+0x1f8/0x330 btrfs_orig_write_end_io+0x1c4/0x2c0 bio_endio+0x63c/0x678 blk_update_request+0x1c4/0xa00 blk_mq_end_request+0x54/0x88 virtblk_request_done+0x124/0x1d0 blk_mq_complete_request+0x84/0xa0 virtblk_done+0x130/0x238 vring_interrupt+0x130/0x288 __handle_irq_event_percpu+0x1e8/0x708 handle_irq_event+0x98/0x1b0 handle_fasteoi_irq+0x264/0x7c0 generic_handle_domain_irq+0xa4/0x108 gic_handle_irq+0x7c/0x1a0 do_interrupt_handler+0xe4/0x148 el1_interrupt+0x30/0x50 el1h_64_irq_handler+0x14/0x20 el1h_64_irq+0x6c/0x70 _raw_spin_unlock_irq+0x38/0x70 __run_timer_base+0xdc/0x5e0 run_timer_softirq+0xa0/0x138 handle_softirqs.llvm.13542289750107964195+0x32c/0xbd0 ____do_softirq.llvm.17674514681856217165+0x18/0x28 call_on_irq_stack+0x24/0x30 __irq_exit_rcu+0x164/0x430 irq_exit_rcu+0x18/0x88 el1_interrupt+0x34/0x50 el1h_64_irq_handler+0x14/0x20 el1h_64_irq+0x6c/0x70 arch_local_irq_enable+0x4/0x8 do_idle+0x1a0/0x3b8 cpu_startup_entry+0x60/0x80 rest_init+0x204/0x228 start_kernel+0x394/0x3f0 __primary_switched+0x8c/0x8958 to a HARDIRQ-irq-unsafe lock: (&eb->refs_lock){+.+.}-{3:3} ... which became HARDIRQ-irq-unsafe at: ... lock_acquire+0x178/0x358 _raw_spin_lock+0x4c/0x68 free_extent_buffer_stale+0x2c/0x170 btrfs_read_sys_array+0x1b0/0x338 open_ctree+0xeb0/0x1df8 btrfs_get_tree+0xb60/0x1110 vfs_get_tree+0x8c/0x250 fc_mount+0x20/0x98 btrfs_get_tree+0x4a4/0x1110 vfs_get_tree+0x8c/0x250 do_new_mount+0x1e0/0x6c0 path_mount+0x4ec/0xa58 __arm64_sys_mount+0x370/0x490 invoke_syscall+0x6c/0x208 el0_svc_common+0x14c/0x1b8 do_el0_svc+0x4c/0x60 el0_svc+0x4c/0x160 el0t_64_sync_handler+0x70/0x100 el0t_64_sync+0x168/0x170 other info that might help us debug this: Possible interrupt unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&eb->refs_lock); local_irq_disable(); lock(&buffer_xa_class); lock(&eb->refs_lock); <Interrupt> lock(&buffer_xa_class); *** DEADLOCK *** 2 locks held by kswapd0/66: #0: ffff800085506e40 (fs_reclaim){+.+.}-{0:0}, at: balance_pgdat+0xe8/0xe50 #1: ffff0000c1d91b88 (&buffer_xa_class){-.-.}-{3:3}, at: try_release_extent_buffer+0x13c/0x560 Link: https://www.kernel.org/doc/Documentation/locking/lockdep-design.rst#:~:text=Multi%2Dlock%20dependency%20rules%3A Fixes: 19d7f65 ("btrfs: convert the buffer_radix to an xarray") CC: stable@vger.kernel.org # 6.16+ Reviewed-by: Boris Burkov <boris@bur.io> Reviewed-by: Qu Wenruo <wqu@suse.com> Signed-off-by: Leo Martins <loemra.dev@gmail.com> Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
@@ -4331,15 +4331,18 @@ static int try_release_subpage_extent_buffer(struct folio *folio)
 	unsigned long end = index + (PAGE_SIZE >> fs_info->nodesize_bits) - 1;
 	int ret;
 
-	xa_lock_irq(&fs_info->buffer_tree);
+	rcu_read_lock();
 	xa_for_each_range(&fs_info->buffer_tree, index, eb, start, end) {
 		/*
 		 * The same as try_release_extent_buffer(), to ensure the eb
 		 * won't disappear out from under us.
 		 */
 		spin_lock(&eb->refs_lock);
+		rcu_read_unlock();
+
 		if (refcount_read(&eb->refs) != 1 || extent_buffer_under_io(eb)) {
 			spin_unlock(&eb->refs_lock);
+			rcu_read_lock();
 			continue;
 		}
 
@@ -4358,11 +4361,10 @@ static int try_release_subpage_extent_buffer(struct folio *folio)
 		 * check the folio private at the end.  And
 		 * release_extent_buffer() will release the refs_lock.
 		 */
-		xa_unlock_irq(&fs_info->buffer_tree);
 		release_extent_buffer(eb);
-		xa_lock_irq(&fs_info->buffer_tree);
+		rcu_read_lock();
 	}
-	xa_unlock_irq(&fs_info->buffer_tree);
+	rcu_read_unlock();
 
 	/*
 	 * Finally to check if we have cleared folio private, as if we have
@@ -4375,7 +4377,6 @@ static int try_release_subpage_extent_buffer(struct folio *folio)
 		ret = 0;
 	spin_unlock(&folio->mapping->i_private_lock);
 	return ret;
-
 }
 
 int try_release_extent_buffer(struct folio *folio)
