[RFC PATCH v2 0/2] ext4: fast commit: fix lockdep issues

Li Chen
2025-12-22T15:19:22Z

Hi,

This series fixes two lockdep issues in the ext4 fast commit paths.

1) ext4_fc_track_inode() can return without sleeping when
EXT4_STATE_FC_COMMITTING is already clear. The lockdep assertion for
i_data_sem should only fire when we actually go to sleep.

2) lockdep reports a possible deadlock due to lock order inversion
between s_fc_lock and i_data_sem. The fast commit writer held s_fc_lock
while writing the fast commit log. Writing the journal inode mapping
can call ext4_map_blocks() and take i_data_sem, while metadata update
paths can hold i_data_sem and call ext4_fc_track_inode() which takes
s_fc_lock.

The fix drops s_fc_lock before the log writing step and uses
EXT4_STATE_FC_COMMITTING to keep inode and create dentry state stable
until cleanup.

Testing:
- QEMU VM, ext4 -O fast_commit on virtio-pmem + dax, verified both lockdep
report reproduces on an older kernel and is gone with this series.

RFC v1 -> RFC v2:
- patch 1: move comments to correct place
- patch 2: add it to patchset.
- add missing RFC prefix

RFC v1: https://lore.kernel.org/linux-ext4/20251222032655.87056-1-me@linux.beauty/T/#u

Li Chen (2):
ext4: fast_commit: assert i_data_sem only before sleep
ext4: fast commit: fix s_fc_lock vs i_data_sem inversion

fs/ext4/fast_commit.c | 96 +++++++++++++++++++++++++++++++------------
1 file changed, 69 insertions(+), 27 deletions(-)

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2.51.0