[libvirt] [PATCH 6/7] Switch to filtering based on log source name instead of filename

[Eric Blake]

On 03/10/2014 04:18 AM, Daniel P. Berrange wrote:
> On Fri, Mar 07, 2014 at 11:05:28AM -0700, Eric Blake wrote:
>> On 03/03/2014 12:18 PM, Daniel P. Berrange wrote:
>>> Both
>>> of these options have a notable performance impact, however, and
>>> it is believed that worst case behaviour where the fields are
>>> read concurrently with being written would merely result in an
>>> mistaken emissions or dropping of the log message in question.
>>> This is an acceptable tradeoff for the performance benefit of
>>> avoiding locking.
>>>
>>
>> Almost.  As long as writes are safe, the worst that can happen is we
>> fail to emit a message that just got enabled, or we emit a message that
>> just got disabled.  But had we used locks to avoid this race, and the
>> locks get obtained in reverse order, we would see the same behavior.  So
>> the locks add no protection, and eliding them in favor of simpler
>> non-atomic integer ops is a safe action.
>>
>> If there were only a single writer, then writes would be automatically
>> safe.  However, you have multiple writers.  Thus, you have the situation
>> that if two writers both try to increment the global serial with no
>> locks or atomic increments, you could end up with the classic symptoms
>> of over- or under-incrementing the global serial.  If three threads compete:
>>

> 
> The writes are already protected by a mutex so I don't think we need
> to use atomic ops for increment either.
> 

> 
> These two functions run with the global log mutex held, so there can
> only be one writer at a time.

Ah, I missed the bigger picture.  Indeed, the increments happen while
the mutex is held, so ACK.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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