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[0/8] Improve futex usage

Message ID 20241225-event-v1-0-a58c8d63eb70@daynix.com (mailing list archive)
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Series Improve futex usage | expand

Message

Akihiko Odaki Dec. 25, 2024, 5:44 a.m. UTC 
In a recent discussion, Phil Dennis-Jordan pointed out a quirk in
QemuEvent destruction due to futex-like abstraction, which prevented
the usage of QemuEvent in new and existing code[1]. With some more
thoughts after this discussion, I also found other problem and room
of improvement in futex usage. Here is a stack of patches to resolve
them.

Patch "futex: Check value after qemu_futex_wait()" ensures
qemu_futex_wait() is used in loops as suggested in the man page.

Patch "futex: Support Windows" implements futex functions for Windows.

Patch "qemu-thread: Avoid futex abstraction for non-Linux" and
"qemu-thread: Use futex for QemuEvent on Windows" enable destroying
QemuEvent immediately after qemu_event_wait().

Patch "qemu-thread: Use futex for QemuEvent on Windows" and
"qemu-thread: Use futex if available for QemuLockCnt" make the use of
futex functions added for Windows.

Patches "migration: Replace QemuSemaphore with QemuEvent",
"migration/colo: Replace QemuSemaphore with QemuEvent", and
"migration/postcopy: Replace QemuSemaphore with QemuEvent" replace some
QemuSemaphores with QemuEvents, which can utilize futex. Some of them
rely on that QemuEvent can be destroyed immediately after
qemu_event_wait().

[1]: https://lore.kernel.org/r/CAAibmn3HZeDeK8FrYhHa1GGwc+N8rBuB2VvMRm7LCt0mUGmsYQ@mail.gmail.com

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
---
Akihiko Odaki (8):
      futex: Check value after qemu_futex_wait()
      futex: Support Windows
      qemu-thread: Avoid futex abstraction for non-Linux
      qemu-thread: Use futex for QemuEvent on Windows
      qemu-thread: Use futex if available for QemuLockCnt
      migration: Replace QemuSemaphore with QemuEvent
      migration/colo: Replace QemuSemaphore with QemuEvent
      migration/postcopy: Replace QemuSemaphore with QemuEvent

 meson.build                       |   2 +
 include/qemu/futex.h              |  43 ++++++++++-
 include/qemu/lockcnt.h            |   2 +-
 include/qemu/thread-posix.h       |   9 ---
 include/qemu/thread-win32.h       |   6 --
 include/qemu/thread.h             |  11 ++-
 migration/migration.h             |  16 ++---
 migration/colo.c                  |  20 +++---
 migration/migration.c             |  33 ++++-----
 migration/postcopy-ram.c          |  10 +--
 migration/savevm.c                |   2 +-
 tests/unit/test-aio-multithread.c |   6 +-
 util/event.c                      | 144 +++++++++++++++++++++++++++++++++++++
 util/lockcnt.c                    |   9 +--
 util/qemu-thread-posix.c          | 148 --------------------------------------
 util/qemu-thread-win32.c          | 129 ---------------------------------
 util/meson.build                  |   3 +-
 17 files changed, 247 insertions(+), 346 deletions(-)
---
base-commit: 34754a3a627e1937be7f3daaa0c5e73d91c7d9b5
change-id: 20241031-event-785a2f0dda4a

Best regards,