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

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

On 09/25/2015 04:21 AM, Wen Congyang wrote:
> On 09/24/2015 08:53 PM, Denis V. Lunev wrote:
>> From: Igor Redko <redkoi@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 <redkoi@virtuozzo.com>
>> Reviewed-by: Anna Melekhova <annam@virtuozzo.com>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Juan Quintela <quintela@redhat.com>
>> CC: Amit Shah <amit.shah@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.

sorry, but there is now very good choice here. We should either
unlock or not call synchronize_rcu which is also an option.

In the other case the rework should be much more sufficient.

Den

>>           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);
>>       }
>>   }
>>