| Message ID | 20231204043911.1273667-1-leo.lilong@huawei.com (mailing list archive) |
|---|---|
| State | Superseded, archived |
| Headers | show |
| Series | [1/2] xfs: add lock protection when remove perag from radix tree | expand |
On Mon, Dec 04, 2023 at 12:39:10PM +0800, Long Li wrote: > Look at the perag insertion into the radix tree, protected by > mp->m_perag_lock. When the file system is unmounted, the perag is > removed from the radix tree, also protected by mp->m_perag_lock. > Therefore, mp->m_perag_lock is also added when removing a perag > from the radix tree in error path in xfs_initialize_perag(). There really can't be anything we are racing with at this point. That beeing said I think clearing locking rules are always a good thing. So maybe reword the above as: "Take mp->m_perag_lock for deletions from the perag radix tree in xfs_initialize_perag to be consistent with additions to it even if there can't be concurrent modifications to it at this point" With that: Reviewed-by: Christoph Hellwig <hch@lst.de>
On Mon, Dec 04, 2023 at 08:35:08PM -0800, Christoph Hellwig wrote: > On Mon, Dec 04, 2023 at 12:39:10PM +0800, Long Li wrote: > > Look at the perag insertion into the radix tree, protected by > > mp->m_perag_lock. When the file system is unmounted, the perag is > > removed from the radix tree, also protected by mp->m_perag_lock. > > Therefore, mp->m_perag_lock is also added when removing a perag > > from the radix tree in error path in xfs_initialize_perag(). > > There really can't be anything we are racing with at this point. I'm pretty sure that there can be racing 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... > That beeing said I think clearing locking rules are always a good > thing. So maybe reword the above as: > > "Take mp->m_perag_lock for deletions from the perag radix tree in > xfs_initialize_perag to be consistent with additions to it even > if there can't be concurrent modifications to it at this point" I don't think it needs even that - just making the radix tree modifications serialise against each other is obviously correct... -Dave.
On Wed, Dec 06, 2023 at 08:28:00AM +1100, Dave Chinner wrote: > On Mon, Dec 04, 2023 at 08:35:08PM -0800, Christoph Hellwig wrote: > > On Mon, Dec 04, 2023 at 12:39:10PM +0800, Long Li wrote: > > > Look at the perag insertion into the radix tree, protected by > > > mp->m_perag_lock. When the file system is unmounted, the perag is > > > removed from the radix tree, also protected by mp->m_perag_lock. > > > Therefore, mp->m_perag_lock is also added when removing a perag > > > from the radix tree in error path in xfs_initialize_perag(). > > > > There really can't be anything we are racing with at this point. > > I'm pretty sure that there can be racing 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... > > > That beeing said I think clearing locking rules are always a good > > thing. So maybe reword the above as: > > > > "Take mp->m_perag_lock for deletions from the perag radix tree in > > xfs_initialize_perag to be consistent with additions to it even > > if there can't be concurrent modifications to it at this point" > > I don't think it needs even that - just making the radix tree > modifications serialise against each other is obviously correct... > If my understanding is correct, it makes sense to add lock protection when modifying the radix tree. So I will update the commit message in the next version. Thanks, Long Li
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);
Look at the perag insertion into the radix tree, protected by mp->m_perag_lock. When the file system is unmounted, the perag is removed from the radix tree, also protected by mp->m_perag_lock. Therefore, mp->m_perag_lock is also added when removing a perag from the radix tree in error path in xfs_initialize_perag(). Signed-off-by: Long Li <leo.lilong@huawei.com> --- fs/xfs/libxfs/xfs_ag.c | 4 ++++ 1 file changed, 4 insertions(+)