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[PATCH v3 0/5] migrations: Fix potential rare race of migration-test aft

From: Peter Xu
Subject: [PATCH v3 0/5] migrations: Fix potential rare race of migration-test after yank
Date: Thu, 22 Jul 2021 13:58:36 -0400

- Use WITH_QEMU_LOCK_GUARD() for patch 2 [Eric]
  (potentially I can also replace other existing uses of qemu_file_lock into
   WITH_QEMU_LOCK_GUARD, but I decided to took Dave's r-b first and leave that
   for later)
- Added r-bs for Dave on patch 2/4
- Add a comment in patch 5 to explain why it's safe to unregister yank without
  qemu_file_lock, in postcopy_pause() [Dave]

- 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:


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         | 57 +++++++++++++++++++++++++++--------
 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, 138 insertions(+), 48 deletions(-)


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