Re: [RFC PATCH v3 3/4] lock to protect memslots

From: Paolo Bonzini <pbonzini@redhat.com>
To: unlisted-recipients:; (no To-header on input)
Cc: Umesh Deshpande <udeshpan@redhat.com>,
	kvm@vger.kernel.org, quintela@redhat.com, mtosatti@redhat.com
Subject: Re: [RFC PATCH v3 3/4] lock to protect memslots
Date: Fri, 12 Aug 2011 08:45:50 +0200	[thread overview]
Message-ID: <4E44CC1E.202@redhat.com> (raw)
In-Reply-To: <4E440131.6020200@redhat.com>

On 08/11/2011 06:20 PM, Paolo Bonzini wrote:
>>
>> +                qemu_mutex_lock_ramlist();
>>                   QLIST_REMOVE(block, next);
>>                   QLIST_INSERT_HEAD(&ram_list.blocks, block, next);
>> +                qemu_mutex_unlock_ramlist();
>
> Theoretically qemu_get_ram_ptr should be protected.  The problem is not
> just accessing the ramlist, it is accessing the data underneath it
> before anyone frees it.  Luckily we can set aside that problem for now,
> because qemu_ram_free_from_ptr is only used by device assignment and
> device assignment makes VMs unmigratable.

Hmm, rethinking about it, all the loops in exec.c should be protected 
from the mutex.  That's not too good because qemu_get_ram_ptr is a hot 
path for TCG.  Perhaps you can also avoid the mutex entirely, and just 
disable the above optimization for most-recently-used-block while 
migration is running.  It's not a complete solution, but it could be 
good enough until we have RAM hot-plug/hot-unplug.

Paolo