[PATCH v3 4/5] btrfs: don't hold dev_replace rwsem over whole of btrfs_map_block

Johannes Thumshirn
2024-07-01T10:25:28Z

From: Johannes Thumshirn <johannes.thumshirn@wdc.com>

Don't hold the dev_replace rwsem for the entirety of btrfs_map_block().

It is only needed to protect
a) calls to find_live_mirror() and
b) calling into handle_ops_on_dev_replace().

But there is no need to hold the rwsem for any kind of set_io_stripe()
calls.

So relax taking the dev_replace rwsem to only protect both cases and check
if the device replace status has changed in the meantime, for which we have
to re-do the find_live_mirror() calls.

This fixes a deadlock on raid-stripe-tree where device replace performs a
scrub operation, which in turn calls into btrfs_map_block() to find the
physical location of the block.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
---
fs/btrfs/volumes.c | 28 +++++++++++++++++-----------
1 file changed, 17 insertions(+), 11 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index fcedc43ef291..4209419244a1 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -6650,14 +6650,9 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
max_len = btrfs_max_io_len(map, map_offset, &io_geom);
*length = min_t(u64, map->chunk_len - map_offset, max_len);

+again:
down_read(&dev_replace->rwsem);
dev_replace_is_ongoing = btrfs_dev_replace_is_ongoing(dev_replace);
- /*
- * Hold the semaphore for read during the whole operation, write is
- * requested at commit time but must wait.
- */
- if (!dev_replace_is_ongoing)
- up_read(&dev_replace->rwsem);

switch (map->type & BTRFS_BLOCK_GROUP_PROFILE_MASK) {
case BTRFS_BLOCK_GROUP_RAID0:
@@ -6695,6 +6690,7 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
"stripe index math went horribly wrong, got stripe_index=%u, num_stripes=%u",
io_geom.stripe_index, map->num_stripes);
ret = -EINVAL;
+ up_read(&dev_replace->rwsem);
goto out;
}

@@ -6710,6 +6706,8 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
*/
num_alloc_stripes += 2;

+ up_read(&dev_replace->rwsem);
+
/*
* If this I/O maps to a single device, try to return the device and
* physical block information on the stack instead of allocating an
@@ -6782,6 +6780,18 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
goto out;
}

+ /*
+ * Check if something changed the dev_replace state since
+ * we've checked it for the last time and if redo the whole
+ * mapping operation.
+ */
+ down_read(&dev_replace->rwsem);
+ if (dev_replace_is_ongoing !=
+ btrfs_dev_replace_is_ongoing(dev_replace)) {
+ up_read(&dev_replace->rwsem);
+ goto again;
+ }
+
if (op != BTRFS_MAP_READ)
io_geom.max_errors = btrfs_chunk_max_errors(map);

@@ -6789,6 +6799,7 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
op != BTRFS_MAP_READ) {
handle_ops_on_dev_replace(bioc, dev_replace, logical, &io_geom);
}
+ up_read(&dev_replace->rwsem);

*bioc_ret = bioc;
bioc->num_stripes = io_geom.num_stripes;
@@ -6796,11 +6807,6 @@ int btrfs_map_block(struct btrfs_fs_info *fs_info, enum btrfs_map_op op,
bioc->mirror_num = io_geom.mirror_num;

out:
- if (dev_replace_is_ongoing) {
- lockdep_assert_held(&dev_replace->rwsem);
- /* Unlock and let waiting writers proceed */
- up_read(&dev_replace->rwsem);
- }
btrfs_free_chunk_map(map);
return ret;
}

--
2.43.0