[RFC PATCH 0/6] exfat: take s_lock in read mode for iomap mapping paths

[Chi Zhiling]

From: Chi Zhiling <chizhiling@xxxxxxxxxx>

The iomap mapping paths currently serialize all inode mappings on the
sbi->s_lock mutex. This series converts it to an rw_semaphore and takes
it in read mode on the mapping paths so that mappings of distinct inodes
proceed in parallel. The preceding patches make the paths safe to run
concurrently: bitmap_lock covers the allocation state, atomic bit ops
cover the volume dirty flag, and the FAT2 mirror copy is serialized
against writeback.

Same-inode access stays serialized by the exclusive inode_lock; writeback
does not hold it but is safe because truncate flushes and truncates the
page cache first and cannot run concurrently with writeback. The lock
ordering (inode_lock -> s_lock -> bitmap_lock) is unchanged, so no new
deadlock scenarios are introduced.

A per-inode read-write lock was also considered, but its implementation
turned out to be considerably more complex, so this series relaxes the
existing s_lock instead. Any suggestions on that approach are welcome.

The series passes the xfstests exfat suite. In Unixbench, the fstime-w
score improved from 3977196.8 to 4746349.6 (~+19%). Any comments and
suggestions are welcome.

Chi Zhiling (6):
  exfat: remove dead hint_bmap updates in I/O and truncate paths
  exfat: take bitmap_lock at the start of exfat_alloc_cluster()
  exfat: use atomic bit ops for volume dirty flag
  exfat: lock FAT2 buffer while copying mirrored FAT entries
  exfat: convert s_lock mutex to rw_semaphore using write lock
  exfat: take s_lock in read mode for iomap mapping paths

 fs/exfat/dir.c       | 14 +++++++-------
 fs/exfat/exfat_fs.h  |  5 ++---
 fs/exfat/exfat_raw.h |  4 ++--
 fs/exfat/fatent.c    | 20 +++++++++++++-------
 fs/exfat/file.c      |  8 ++------
 fs/exfat/inode.c     | 15 +++++----------
 fs/exfat/iomap.c     |  4 ++--
 fs/exfat/namei.c     | 26 +++++++++++++-------------
 fs/exfat/super.c     | 42 +++++++++++++++++++++---------------------
 9 files changed, 67 insertions(+), 71 deletions(-)

-- 
2.53.0