On 09/24/2015 08:53 PM, Denis V. Lunev wrote: > 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();
migration_end() can called in two cases: 1. migration_completed 2. migration is cancelled In case 1, you should not unlock iothread, otherwise, the vm's state may be changed unexpectedly. > 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(); Hmm, I think it is OK to unlock iothread here > g_free(old_bitmap); > } > } >