Re: [Qemu-devel] [PATCH] fixup! block-migration: acquire AioContext as necessary

[Paolo Bonzini <pbonzini@redhat.com>]

On 23/02/2016 06:39, Fam Zheng wrote:
> BTW, could you also explain the blk_mig_lock() question (*) I had?
> 
> *: https://lists.gnu.org/archive/html/qemu-devel/2016-02/msg03317.html

Sorry, missed that:

> 
>> @@ -597,21 +627,28 @@ static void block_migration_cleanup(void *opaque)
>>  {
>>      BlkMigDevState *bmds;
>>      BlkMigBlock *blk;
>> +    AioContext *ctx;
>>  
>>      bdrv_drain_all();
>>  
>>      unset_dirty_tracking();
>>  
>> -    blk_mig_lock();
> 
> Why is it okay to skip the blk_mig_lock() for block_mig_state.bmds_list?

The bmds_list is not protected by blk_mig_lock:

    /* Written during setup phase.  Can be read without a lock.  */
    int blk_enable;
    int shared_base;
    QSIMPLEQ_HEAD(bmds_list, BlkMigDevState) bmds_list;
    int64_t total_sector_sum;
    bool zero_blocks;

block_migration_cleanup is called from qemu_savevm_state_cleanup, when
there can be no concurrency between the migration thread and the I/O
thread.  In fact the call of qemu_savevm_state_cleanup might as well be
moved out of the migration thread.

Paolo