Re: [PATCH v3 1/2] xfs: add lock protection when remove perag from radix tree

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Dec 13, 2023 at 11:10:12AM +0800, Long Li wrote:
> 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>

Looks correct to me, and I didn't find any other suspicious accesses of
m_perag_tree so

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

--D

> ---
>  fs/xfs/libxfs/xfs_ag.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/fs/xfs/libxfs/xfs_ag.c b/fs/xfs/libxfs/xfs_ag.c
> index f62ff125a50a..c730976fdfc0 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);
> -- 
> 2.31.1
> 
>