[RFC,0/1] remove redundant q->sysfs_dir_lock

Message

Nilay Shroff Jan. 20, 2025, 1:04 p.m. UTC

Hi,

In the current implementation we use q->sysfs_dir_lock for protecting
kobject addition/deletion while we register/unregister blk-mq with
sysfs. However the sysfs/kernfs internal implementation already protects
against the simultaneous addtion/deletion of kobjects. So in that sense
use of q->sysfs_dir_lock appears redundant.

Furthermore, there're few other callsites in the current code where we
use q->sysfs_dir_lock along with q->sysfs_lock while addition/deletion of
independent access ranges for a disk under sysfs. Please refer, disk_
register_independent_access_ranges() and disk_unregister_independent_
access_ranges(). Here as well we could easily remove use of q->sysfs_dir_
lock.

The only thing which q->syfs_dir_lock appears to protect is the use of
variable q->mq_sysfs_init_done. However this could be solved by converting
q->mq_sysfs_init_done to an atomic variable.

In past few days, we have seen many lockdep splat in block layer and
getting rid of this one might help reduce some contention as well we
need to worry less about lock ordering wrt to this lock.

Nilay Shroff (1):
  block: get rid of request queue ->sysfs_dir_lock

 block/blk-core.c       |  1 -
 block/blk-ia-ranges.c  |  4 ----
 block/blk-mq-sysfs.c   | 25 +++++++------------------
 block/blk-sysfs.c      |  5 -----
 include/linux/blkdev.h |  3 +--
 5 files changed, 8 insertions(+), 30 deletions(-)