[v2,0/5] migrations: Fix potential rare race of migration-test after yank

Message

Peter Xu July 21, 2021, 7:34 p.m. UTC

v2:
- Pick r-b for Dave on patch 1/3
- Move migration_file_get_ioc() from patch 5 to patch 4, meanwhile rename it to
  qemu_file_get_ioc(). [Dave]
- Patch 2 "migration: Shutdown src in await_return_path_close_on_source()" is
  replaced by patch "migration: Make from_dst_file accesses thread-safe" [Dave]

Patch 1 fixes a possible race that migration thread can accidentally skip
join() of rp_thread even if the return thread is enabled.  Patch 1 is suspected
to also be the root cause of the recent hard-to-reproduce migration-test
failure here reported by PMM:

https://lore.kernel.org/qemu-devel/YPamXAHwan%2FPPXLf@work-vm/

I didn't reproduce it myself; but after co-debugged with Dave it's suspected
that the race of rp_thread could be the cause.  It's not exposed before because
yank is soo strict on releasing instances, while we're not that strict before,
and didn't join() on rp_thread wasn't so dangerous after all when migration
succeeded before.

Patch 2 fixes another theoretical race on accessing from_dst_file spotted by
Dave.  I don't think there's known issues with it, but may still worth fixing.

Patch 3 should be a cleanup on yank that I think would be nice to have.

Patch 4-5 are further cleanups to remove the ref==1 check in channel_close(),
finally, as I always thought that's a bit hackish.  So I used explicit
unregister of the yank function at necessary places to replace that ref==1 one.

I still think having patch 3-5 altogether would be great, however I think patch
1 should still be the most important to be reviewed.  Also it would be great to
know whether patch 1 could fix the known yank crash.

Please review, thanks.

Peter Xu (5):
  migration: Fix missing join() of rp_thread
  migration: Make from_dst_file accesses thread-safe
  migration: Introduce migration_ioc_[un]register_yank()
  migration: Teach QEMUFile to be QIOChannel-aware
  migration: Move the yank unregister of channel_close out

 migration/channel.c           | 15 ++-----------
 migration/migration.c         | 41 +++++++++++++++++++++++++++-------
 migration/migration.h         | 15 ++++++++++---
 migration/multifd.c           |  8 ++-----
 migration/qemu-file-channel.c | 11 ++-------
 migration/qemu-file.c         | 17 +++++++++++++-
 migration/qemu-file.h         |  4 +++-
 migration/ram.c               |  3 ++-
 migration/savevm.c            | 11 +++++++--
 migration/yank_functions.c    | 42 +++++++++++++++++++++++++++++++++++
 migration/yank_functions.h    |  3 +++
 11 files changed, 126 insertions(+), 44 deletions(-)