[PATCH v3 0/4] io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER

Caleb Sander Mateos
2025-11-25T23:39:40Z

Setting IORING_SETUP_SINGLE_ISSUER when creating an io_uring doesn't
actually enable any additional optimizations (aside from being a
requirement for IORING_SETUP_DEFER_TASKRUN). This series leverages
IORING_SETUP_SINGLE_ISSUER's guarantee that only one task submits SQEs
to skip taking the uring_lock mutex for the issue and task work paths.

First, we need to disable this optimization for IORING_SETUP_SQPOLL by
clearing the IORING_SETUP_SINGLE_ISSUER flag. For IORING_SETUP_SQPOLL,
the SQ thread is the one taking the uring_lock mutex in the issue path.
Since concurrent io_uring_register() syscalls are allowed on the thread
that created/enabled the io_uring, some additional synchronization
method would be required to synchronize the two threads. This is
possible in principle by having io_uring_register() schedule a task work
item to suspend the SQ thread, but this seems very complex for a niche
use case.

Then we factor out helpers for interacting with uring_lock to centralize
the logic.

Finally, we implement the optimization for IORING_SETUP_SINGLE_ISSUER.
If the io_ring_ctx is setup with IORING_SETUP_SINGLE_ISSUER, skip the
uring_lock mutex_lock() and mutex_unlock() on the submitter_task. On
other tasks acquiring the ctx uring lock, use a task work item to
suspend the submitter_task for the critical section.
In io_uring_register(), continue to always acquire the uring_lock mutex.
io_uring_register() can be called on a disabled io_ring_ctx (indeed,
it's required to enable it), when submitter_task isn't set yet. After
submitter_task is set, io_uring_register() is only permitted on
submitter_task, so uring_lock suffices to exclude all other users.

v3:
- Ensure mutual exclusion on threads other than submitter_task via a
task work item to suspend submitter_task
- Drop patches already merged

v2:
- Don't enable these optimizations for IORING_SETUP_SQPOLL, as we still
need to synchronize SQ thread submission with io_uring_register()

Caleb Sander Mateos (4):
io_uring: clear IORING_SETUP_SINGLE_ISSUER for IORING_SETUP_SQPOLL
io_uring: use io_ring_submit_lock() in io_iopoll_req_issued()
io_uring: factor out uring_lock helpers
io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER

include/linux/io_uring_types.h | 12 +-
io_uring/cancel.c | 36 +++---
io_uring/eventfd.c | 5 +-
io_uring/fdinfo.c | 6 +-
io_uring/filetable.c | 8 +-
io_uring/futex.c | 14 ++-
io_uring/io_uring.c | 205 ++++++++++++++++++++-------------
io_uring/io_uring.h | 204 +++++++++++++++++++++++++++++---
io_uring/kbuf.c | 38 +++---
io_uring/memmap.h | 2 +-
io_uring/msg_ring.c | 29 +++--
io_uring/notif.c | 5 +-
io_uring/notif.h | 3 +-
io_uring/openclose.c | 14 ++-
io_uring/poll.c | 21 ++--
io_uring/register.c | 34 +++---
io_uring/rsrc.c | 37 +++---
io_uring/rsrc.h | 3 +-
io_uring/rw.c | 2 +-
io_uring/splice.c | 5 +-
io_uring/sqpoll.c | 5 +-
io_uring/tctx.c | 24 ++--
io_uring/uring_cmd.c | 13 ++-
io_uring/waitid.c | 13 ++-
io_uring/zcrx.c | 2 +-
25 files changed, 506 insertions(+), 234 deletions(-)

--
2.45.2