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

[Avi Kivity <avi@redhat.com>]

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.

> 
>> 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.

> 
> 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.

> And at the same time, mr->subregions is
> exposed under SMP without big lock.
> 

Who accesses it?

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



-- 
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