From: Igor Redko <red...@virtuozzo.com> 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 The patch just releases QGM before calling synchronize_rcu(). Signed-off-by: Igor Redko <red...@virtuozzo.com> Reviewed-by: Anna Melekhova <an...@virtuozzo.com> Signed-off-by: Denis V. Lunev <d...@openvz.org> CC: Juan Quintela <quint...@redhat.com> CC: Amit Shah <amit.s...@redhat.com> --- migration/ram.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/migration/ram.c b/migration/ram.c index 7f007e6..d01febc 100644 --- a/migration/ram.c +++ b/migration/ram.c @@ -1028,12 +1028,16 @@ static void migration_end(void) { /* caller have hold iothread lock or is in a bh, so there is * no writing race against this migration_bitmap + * but rcu used not only for migration_bitmap, so we should + * release QGM or we get in deadlock. */ unsigned long *bitmap = migration_bitmap; atomic_rcu_set(&migration_bitmap, NULL); if (bitmap) { memory_global_dirty_log_stop(); + qemu_mutex_unlock_iothread(); synchronize_rcu(); + qemu_mutex_lock_iothread(); g_free(bitmap); } @@ -1085,7 +1089,9 @@ void migration_bitmap_extend(ram_addr_t old, ram_addr_t new) atomic_rcu_set(&migration_bitmap, bitmap); qemu_mutex_unlock(&migration_bitmap_mutex); migration_dirty_pages += new - old; + qemu_mutex_unlock_iothread(); synchronize_rcu(); + qemu_mutex_lock_iothread(); g_free(old_bitmap); } } -- 2.1.4