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

[Lukas Straub]

On Thu, 22 Jul 2021 13:58:36 -0400
Peter Xu <peterx@redhat.com> wrote:

> v3:
> - 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]
> 
> 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         | 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(-)
> 

Looks good to me,
Reviewed-by: Lukas Straub <lukasstraub2@web.de>

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