[Qemu-commits] [qemu/qemu] cf02f2: migration: Fix race that dest preempt thread close...

[Alex Bennée]

  Branch: refs/heads/staging
  Home:   https://github.com/qemu/qemu
  Commit: cf02f29e1e3843784630d04783e372fa541a77e5
      
https://github.com/qemu/qemu/commit/cf02f29e1e3843784630d04783e372fa541a77e5
  Author: Peter Xu <peterx@redhat.com>
  Date:   2023-09-27 (Wed, 27 Sep 2023)

  Changed paths:
    M migration/migration.c
    M migration/migration.h
    M migration/postcopy-ram.c

  Log Message:
  -----------
  migration: Fix race that dest preempt thread close too early

We hit intermit CI issue on failing at migration-test over the unit test
preempt/plain:

qemu-system-x86_64: Unable to read from socket: Connection reset by peer
Memory content inconsistency at 5b43000 first_byte = bd last_byte = bc current 
= 4f hit_edge = 1
**
ERROR:../tests/qtest/migration-test.c:300:check_guests_ram: assertion failed: 
(bad == 0)
(test program exited with status code -6)

Fabiano debugged into it and found that the preempt thread can quit even
without receiving all the pages, which can cause guest not receiving all
the pages and corrupt the guest memory.

To make sure preempt thread finished receiving all the pages, we can rely
on the page_requested_count being zero because preempt channel will only
receive requested page faults. Note, not all the faulted pages are required
to be sent via the preempt channel/thread; imagine the case when a
requested page is just queued into the background main channel for
migration, the src qemu will just still send it via the background channel.

Here instead of spinning over reading the count, we add a condvar so the
main thread can wait on it if that unusual case happened, without burning
the cpu for no good reason, even if the duration is short; so even if we
spin in this rare case is probably fine.  It's just better to not do so.

The condvar is only used when that special case is triggered.  Some memory
ordering trick is needed to guarantee it from happening (against the
preempt thread status field), so the main thread will always get a kick
when that triggers correctly.

Closes: https://gitlab.com/qemu-project/qemu/-/issues/1886
Debugged-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-2-farosas@suse.de>


  Commit: 28a8347281e24c2e7bba6d3301472eda41d4c096
      
https://github.com/qemu/qemu/commit/28a8347281e24c2e7bba6d3301472eda41d4c096
  Author: Fabiano Rosas <farosas@suse.de>
  Date:   2023-09-27 (Wed, 27 Sep 2023)

  Changed paths:
    M migration/migration.c

  Log Message:
  -----------
  migration: Fix possible race when setting rp_state.error

We don't need to set the rp_state.error right after a shutdown because
qemu_file_shutdown() always sets the QEMUFile error, so the return
path thread would have seen it and set the rp error itself.

Setting the error outside of the thread is also racy because the
thread could clear it after we set it.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-3-farosas@suse.de>


  Commit: 639decf529793fc544c8055b82be8abe77fa48fa
      
https://github.com/qemu/qemu/commit/639decf529793fc544c8055b82be8abe77fa48fa
  Author: Fabiano Rosas <farosas@suse.de>
  Date:   2023-09-27 (Wed, 27 Sep 2023)

  Changed paths:
    M migration/migration.c

  Log Message:
  -----------
  migration: Fix possible races when shutting down the return path

We cannot call qemu_file_shutdown() on the return path file without
taking the file lock. The return path thread could be running it's
cleanup code and have just cleared the from_dst_file pointer.

Checking ms->to_dst_file for errors could also race with
migrate_fd_cleanup() which clears the to_dst_file pointer.

Protect both accesses by taking the file lock.

This was caught by inspection, it should be rare, but the next patches
will start calling this code from other places, so let's do the
correct thing.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-4-farosas@suse.de>


  Commit: 7478fb0df914f0a5ab551ff74b1df62dd250500e
      
https://github.com/qemu/qemu/commit/7478fb0df914f0a5ab551ff74b1df62dd250500e
  Author: Fabiano Rosas <farosas@suse.de>
  Date:   2023-09-27 (Wed, 27 Sep 2023)

  Changed paths:
    M migration/migration.c

  Log Message:
  -----------
  migration: Fix possible race when shutting down to_dst_file

It's not safe to call qemu_file_shutdown() on the to_dst_file without
first checking for the file's presence under the lock. The cleanup of
this file happens at postcopy_pause() and migrate_fd_cleanup() which
are not necessarily running in the same thread as migrate_fd_cancel().

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-5-farosas@suse.de>


  Commit: b3b101157d4651f12e6b3361af2de6bace7f9b4a
      
https://github.com/qemu/qemu/commit/b3b101157d4651f12e6b3361af2de6bace7f9b4a
  Author: Fabiano Rosas <farosas@suse.de>
  Date:   2023-09-27 (Wed, 27 Sep 2023)

  Changed paths:
    M migration/migration.c

  Log Message:
  -----------
  migration: Remove redundant cleanup of postcopy_qemufile_src

This file is owned by the return path thread which is already doing
cleanup.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-6-farosas@suse.de>


  Commit: d50f5dc075cbb891bfe4a9378600a4871264468a
      
https://github.com/qemu/qemu/commit/d50f5dc075cbb891bfe4a9378600a4871264468a
  Author: Fabiano Rosas <farosas@suse.de>
  Date:   2023-09-27 (Wed, 27 Sep 2023)

  Changed paths:
    M migration/migration.c

  Log Message:
  -----------
  migration: Consolidate return path closing code

We'll start calling the await_return_path_close_on_source() function
from other parts of the code, so move all of the related checks and
tracepoints into it.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-7-farosas@suse.de>


  Commit: ef796ee93b313ed2f0b427ef30320417387d2ad5
      
https://github.com/qemu/qemu/commit/ef796ee93b313ed2f0b427ef30320417387d2ad5
  Author: Fabiano Rosas <farosas@suse.de>
  Date:   2023-09-27 (Wed, 27 Sep 2023)

  Changed paths:
    M migration/migration.c
    M migration/migration.h

  Log Message:
  -----------
  migration: Replace the return path retry logic

Replace the return path retry logic with finishing and restarting the
thread. This fixes a race when resuming the migration that leads to a
segfault.

Currently when doing postcopy we consider that an IO error on the
return path file could be due to a network intermittency. We then keep
the thread alive but have it do cleanup of the 'from_dst_file' and
wait on the 'postcopy_pause_rp' semaphore. When the user issues a
migrate resume, a new return path is opened and the thread is allowed
to continue.

There's a race condition in the above mechanism. It is possible for
the new return path file to be setup *before* the cleanup code in the
return path thread has had a chance to run, leading to the *new* file
being closed and the pointer set to NULL. When the thread is released
after the resume, it tries to dereference 'from_dst_file' and crashes:

Thread 7 "return path" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fffd1dbf700 (LWP 9611)]
0x00005555560e4893 in qemu_file_get_error_obj (f=0x0, errp=0x0) at 
../migration/qemu-file.c:154
154         return f->last_error;

(gdb) bt
 #0  0x00005555560e4893 in qemu_file_get_error_obj (f=0x0, errp=0x0) at 
../migration/qemu-file.c:154
 #1  0x00005555560e4983 in qemu_file_get_error (f=0x0) at 
../migration/qemu-file.c:206
 #2  0x0000555555b9a1df in source_return_path_thread (opaque=0x555556e06000) at 
../migration/migration.c:1876
 #3  0x000055555602e14f in qemu_thread_start (args=0x55555782e780) at 
../util/qemu-thread-posix.c:541
 #4  0x00007ffff38d76ea in start_thread (arg=0x7fffd1dbf700) at 
pthread_create.c:477
 #5  0x00007ffff35efa6f in clone () at 
../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Here's the race (important bit is open_return_path happening before
migration_release_dst_files):

migration                 | qmp                         | return path
--------------------------+-----------------------------+---------------------------------
                            qmp_migrate_pause()
                             shutdown(ms->to_dst_file)
                              f->last_error = -EIO
migrate_detect_error()
 postcopy_pause()
  set_state(PAUSED)
  wait(postcopy_pause_sem)
                            qmp_migrate(resume)
                            migrate_fd_connect()
                             resume = state == PAUSED
                             open_return_path <-- TOO SOON!
                             set_state(RECOVER)
                             post(postcopy_pause_sem)
                                                        (incoming closes 
to_src_file)
                                                        res = 
qemu_file_get_error(rp)
                                                        
migration_release_dst_files()
                                                        
ms->rp_state.from_dst_file = NULL
  post(postcopy_pause_rp_sem)
                                                        
postcopy_pause_return_path_thread()
                                                          
wait(postcopy_pause_rp_sem)
                                                        rp = 
ms->rp_state.from_dst_file
                                                        goto retry
                                                        qemu_file_get_error(rp)
                                                        SIGSEGV
-------------------------------------------------------------------------------------------

We can keep the retry logic without having the thread alive and
waiting. The only piece of data used by it is the 'from_dst_file' and
it is only allowed to proceed after a migrate resume is issued and the
semaphore released at migrate_fd_connect().

Move the retry logic to outside the thread by waiting for the thread
to finish before pausing the migration.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-8-farosas@suse.de>


  Commit: 36e9aab3c569d4c9ad780473596e18479838d1aa
      
https://github.com/qemu/qemu/commit/36e9aab3c569d4c9ad780473596e18479838d1aa
  Author: Fabiano Rosas <farosas@suse.de>
  Date:   2023-09-27 (Wed, 27 Sep 2023)

  Changed paths:
    M migration/migration.c

  Log Message:
  -----------
  migration: Move return path cleanup to main migration thread

Now that the return path thread is allowed to finish during a paused
migration, we can move the cleanup of the QEMUFiles to the main
migration thread.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-9-farosas@suse.de>


Compare: https://github.com/qemu/qemu/compare/5dfd80e38b63...36e9aab3c569