[Qemu-devel] [PATCH 1/1] migration: fix deadlock

[Denis V. Lunev]

From: Igor Redko <address@hidden>

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 <address@hidden>
Reviewed-by: Anna Melekhova <address@hidden>
Signed-off-by: Denis V. Lunev <address@hidden>
CC: Juan Quintela <address@hidden>
CC: Amit Shah <address@hidden>
---
 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