Re: [Qemu-devel] [PATCH 04/15] memory: MemoryRegion topology must be stable when updating

[liu ping fan]

On Thu, Aug 9, 2012 at 4:24 PM, Avi Kivity <address@hidden> wrote:
> On 08/09/2012 10:28 AM, liu ping fan wrote:
>>>
>>> Seems to me that nothing in memory.c can susceptible to races.  It must
>>> already be called under the big qemu lock, and with the exception of
>>> mutators (memory_region_set_*), changes aren't directly visible.
>>>
>> Yes, what I want to do is "prepare unplug out of protection of global
>> lock".  When io-dispatch and mmio-dispatch are all out of big lock, we
>> will run into the following scene:
>>     In vcpu context A, qdev_unplug_complete()-> delete subregion;
>>     In context B, write pci bar --> pci mapping update    -> add subregion
>
> Why do you want unlocked unplug?  Unplug is rare and complicated; there
> are no performance considerations on one hand, and difficulty of testing
> for lock correctness on the other.  I think it is better if it remains
> protected by the global lock.
>
Oh, yes! I deviate quite far from the origin aim, and introduce some
unnecessary complicate.

>>
>>> I think it's sufficient to take the mem_map_lock at the beginning of
>>> core_begin() and drop it at the end of core_commit().  That means all
>>> updates of volatile state, phys_map, are protected.
>>>
>> The mem_map_lock is to protect both address_space_io and
>> address_space_memory. When without the protection of big lock,
>> competing will raise among the updaters
>> (memory_region_{add,del}_subregion and the readers
>> generate_memory_topology()->render_memory_region().
>
> These should all run under the big qemu lock, for the same reasons.
> They are rare and not performance sensitive.  Only phys_map reads are
> performance sensitive.
>
OK, I see. Leave the big lock as it is, except for mmio, we will not
worry about it.
>>
>> If just in core_begin/commit, we will duplicate it for
>> xx_begin/commit, right?
>
> No.  Other listeners will be protected by the global lock.
>
Yes, if leave the big lock as it is.
>> And at the same time, mr->subregions is
>> exposed under SMP without big lock.
>>
>
> Who accesses it?
>
Again, I assume the updaters out of the protection of the big lock

> IMO locking should look like:
>
>   phys_map: mem_map_lock
>   dispatch callbacks: device specific lock (or big qemu lock for
> unconverted devices)
>   everything else: big qemu lock
>
I See. Thank you for the review. And I will eliminate the unnecessary
complicate and effort for the next version

Regards,
pingfan
>
>
> --
> error compiling committee.c: too many arguments to function