[Qemu-devel] [PATCH v2 0/2] migration: fix deadlock

["Denis V. Lunev" <den@openvz.org>]

Release qemu global mutex before call synchronize_rcu().
synchronize_rcu() waiting for all readers to finish their critical
sections. There is at least one critical section in which we try
to get QGM (critical section is in address_space_rw() and
prepare_mmio_access() is trying to aquire QGM).

Both functions (migration_end() and migration_bitmap_extend())
are called from main thread which is holding QGM.

Thus there is a race condition that ends up with deadlock:
main thread     working thread
Lock QGA                |
|             Call KVM_EXIT_IO handler
|                       |
|        Open rcu reader's critical section
Migration cleanup bh    |
|                       |
synchronize_rcu() is    |
waiting for readers     |
|            prepare_mmio_access() is waiting for QGM
  \                   /
         deadlock

Patches here are quick and dirty, compile-tested only to validate the
architectual approach.

Igor, Anna, can you pls start your tests with these patches instead of your
original one. Thank you.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Igor Redko <redkoi@virtuozzo.com>
CC: Anna Melekhova <annam@virtuozzo.com>
CC: Juan Quintela <quintela@redhat.com>
CC: Amit Shah <amit.shah@redhat.com>

Denis V. Lunev (2):
  migration: bitmap_set is unnecessary as bitmap_new uses g_try_malloc0
  migration: fix deadlock

 migration/ram.c | 45 ++++++++++++++++++++++++++++-----------------
 1 file changed, 28 insertions(+), 17 deletions(-)

-- 
2.1.4