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  • Anton Anton anjo
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  1. Sep 29, 2023
  3. Sep 27, 2023
    • Fabiano Rosas's avatar
      migration: Move return path cleanup to main migration thread · 36e9aab3
      Fabiano Rosas authored 
      
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: default avatarPeter Xu <peterx@redhat.com>
Signed-off-by: default avatarFabiano Rosas <farosas@suse.de>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-9-farosas@suse.de>
      36e9aab3
    • Fabiano Rosas's avatar
      migration: Replace the return path retry logic · ef796ee9
      Fabiano Rosas authored 
      
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: default avatarPeter Xu <peterx@redhat.com>
Signed-off-by: default avatarFabiano Rosas <farosas@suse.de>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-8-farosas@suse.de>
      ef796ee9
    • Fabiano Rosas's avatar
      migration: Consolidate return path closing code · d50f5dc0
      Fabiano Rosas authored 
      
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: default avatarPeter Xu <peterx@redhat.com>
Signed-off-by: default avatarFabiano Rosas <farosas@suse.de>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-7-farosas@suse.de>
      d50f5dc0
    • Fabiano Rosas's avatar
      migration: Remove redundant cleanup of postcopy_qemufile_src · b3b10115
      Fabiano Rosas authored 
      
This file is owned by the return path thread which is already doing
cleanup.

Reviewed-by: default avatarPeter Xu <peterx@redhat.com>
Signed-off-by: default avatarFabiano Rosas <farosas@suse.de>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-6-farosas@suse.de>
      b3b10115
    • Fabiano Rosas's avatar
      migration: Fix possible race when shutting down to_dst_file · 7478fb0d
      Fabiano Rosas authored 
      
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: default avatarPeter Xu <peterx@redhat.com>
Signed-off-by: default avatarFabiano Rosas <farosas@suse.de>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-5-farosas@suse.de>
      7478fb0d
    • Fabiano Rosas's avatar
      migration: Fix possible races when shutting down the return path · 639decf5
      Fabiano Rosas authored 
      
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: default avatarPeter Xu <peterx@redhat.com>
Signed-off-by: default avatarFabiano Rosas <farosas@suse.de>
Signed-off-by: default avatarStefan Hajnoczi <stefanha@redhat.com>
Message-ID: <20230918172822.19052-4-farosas@suse.de>
      639decf5