Re: [Qemu-devel] [RFC PATCH v4 4/5] separate thread for VM migration

[Paolo Bonzini]

On 08/16/2011 08:56 PM, Umesh Deshpande wrote:
> +            qemu_mutex_lock_ramlist();

Taken locks: iothread, ramlist

> +            qemu_mutex_unlock_iothread();

Taken locks: ramlist

> +            s->wait_for_unfreeze(s);
> +            qemu_mutex_lock_iothread();

Taken locks: ramlist, iothread

You'd have a deadlock here if hypothetically you had two migrations at 
the same time.

> +            qemu_mutex_unlock_ramlist();

But in general, why this locking?  The buffered file need not know 
anything about the ram list and its mutex.  Only ram_save_live needs to 
hold the ramlist lock---starting just before sort_ram_list and ending 
just after the end of stage 3.  That should be part of patch 2.

Actually buffered_file.c should ideally not even take the iothread lock! 
 The code there is only copying data from a private buffer to a file 
descriptor; neither is shared.  It's migrate_fd_put_buffer that should 
take care of locking.  This is an example of why keeping the separation 
of QEMUBufferedFile is a good idea at least for now.

I am still kind of unconvinced about calling qemu_fclose from the 
migration thread.  You still have one instance of cancellation in the 
iothread when migrate_fd_release is called.  Ideally, as soon as 
migration finishes or has an error you could trigger a bottom half that 
closes the file (which in turn joins the thread).  Migration state 
notifiers should also be run only from the iothread.  Failure to do so 
(or in general lack of a policy of what runs where) can lead to very 
difficult bugs.  Not so much hard to debug in this case (we have a 
global lock, so things cannot go _that_ bad), but hard to fix without 
redoing everything.

However, this patch is a good start (with locking fixed).  It should 
takes several incremental steps before getting there, including 
incredible simplification if you take into account that migration can 
block and wait_for_unfreeze can disappear.  In the end it probably 
should be committed as a single patch, but I'm liking the patches more 
and more.

Paolo