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[v2,1/3] xfs: add lock protection when remove perag from radix tree

Message ID 20231209122107.2422441-1-leo.lilong@huawei.com (mailing list archive)
State Superseded
Headers show
Series [v2,1/3] xfs: add lock protection when remove perag from radix tree | expand

Commit Message

Long Li Dec. 9, 2023, 12:21 p.m. UTC 
Take mp->m_perag_lock for deletions from the perag radix tree in
xfs_initialize_perag to prevent racing with tagging operations.
Lookups are fine - they are RCU protected so already deal with the
tree changing shape underneath the lookup - but tagging operations
require the tree to be stable while the tags are propagated back up
to the root.

Right now there's nothing stopping radix tree tagging from operating
while a growfs operation is progress and adding/removing new entries
into the radix tree.

Hence we can have traversals that require a stable tree occurring at
the same time we are removing unused entries from the radix tree which
causes the shape of the tree to change.

Likely this hasn't caused a problem in the past because we are only
doing append addition and removal so the active AG part of the tree
is not changing shape, but that doesn't mean it is safe. Just making
the radix tree modifications serialise against each other is obviously
correct.

Signed-off-by: Long Li <leo.lilong@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
---
v2:
- Refer to Dave's explanation and rewrite the commit message.
 fs/xfs/libxfs/xfs_ag.c | 4 ++++
 1 file changed, 4 insertions(+)
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Patch

diff --git a/fs/xfs/libxfs/xfs_ag.c b/fs/xfs/libxfs/xfs_ag.c
index f9f4d694640d..cc10a3ca052f 100644
--- a/fs/xfs/libxfs/xfs_ag.c
+++ b/fs/xfs/libxfs/xfs_ag.c
@@ -424,13 +424,17 @@  xfs_initialize_perag(
 
 out_remove_pag:
 	xfs_defer_drain_free(&pag->pag_intents_drain);
+	spin_lock(&mp->m_perag_lock);
 	radix_tree_delete(&mp->m_perag_tree, index);
+	spin_unlock(&mp->m_perag_lock);
 out_free_pag:
 	kmem_free(pag);
 out_unwind_new_pags:
 	/* unwind any prior newly initialized pags */
 	for (index = first_initialised; index < agcount; index++) {
+		spin_lock(&mp->m_perag_lock);
 		pag = radix_tree_delete(&mp->m_perag_tree, index);
+		spin_unlock(&mp->m_perag_lock);
 		if (!pag)
 			break;
 		xfs_buf_hash_destroy(pag);