[PATCH] Potential reference count races

Jacek Caban jacek at codeweavers.com
Mon Oct 29 03:30:30 CDT 2012

On 10/29/12 01:25, Max TenEyck Woodbury wrote:
> On 10/28/2012 12:06 PM, Nikolay Sivov wrote:
>> On 10/28/2012 17:44, Max TenEyck Woodbury wrote:
>>> On 10/28/2012 02:40 AM, Nikolay Sivov wrote:
>>>> On 10/28/2012 04:59, max at mtew.isa-geek.net wrote:
>>>>> From: Max TenEyck Woodbury <max at mtew.isa-geek.net>
>>>>> I have been looking at the Microsoft COM and related documentations
>>>>> and noticed that they emphatically recommend using the Interlocked...
>>>>> functions when manipulating reference counts.  I managed to set up a
>>>>> search that showed where many of the reference count updates occur and
>>>>> was somewhat surprised at how often this advice was not followed.
>>>> It doesn't mean it always has to be followed.
>>> True in a limited sense, but there is a good reason behind their
>>> recommendation.  Unless there is a good reason not to do so, this
>>> particular piece of advice should be followed.
>> COM objects in wine follow this recommendation in general, even object
>> itself is not thread safe.
>> This doesn't mean however that you need this every time you have some
>> kind of refcount of any sort.
> It may or may not be necessary every time, but it should be demonstrated
> that it is not necessary rather than assumed that it is not.  This is a
> 'race condition' after all, and they are known to be rare and difficult
> to isolate.  I think it is good practice to assume there could be a race
> problem rather than otherwise.

No, it's a good practice to understand what you're changing instead of
blindly assuming that everything with 'ref' in its name needs
Interlocked* functions. You're patch even changes things in jscript that
are not COM objecst, in a middle of code that is not thread safe anyway.
It's documented that those parts of the code are not thread safe. Even
worse, you were changing lines that even have a comment about no need
for atomic operations (thanks for Michael, who predicted that someone
may send patches like you).

>>>>> While I have not converted every reference count update to use the
>>>>> Interlocked... functions, this set of patches fixes a fair number of
>>>>> them.
>>>>> These are not associated with any particular bug report; they are
>>>>> simply a general precaution against operations that are known to be
>>>>> associated with race conditions.
>>>> This precaution doesn't work in general. It's not enough to atomically
>>>> update refcount to make an implementation thread safe. Also not
>>>> everything is supposed to be thread safe in a first place.
>>> First, explain what does not have to be thread safe.
>> Anything really, COM objects in particular if you were talking about them.
> I think you are talking about the apartment model here, which forces
> thread serialization.  Despite that, the Microsoft documentation still
> recommends interlocked operations on reference counts...

Just to make it clear: using Interlocked* functions in generic COM
object is a good practice. But lack of it is not always a bug and you
shouldn't change it without understanding what you are doing.

>>> I believe that application may try to use multiple threads anywhere,
>>> so everything that can be made thread safe, should be.
>> No.
> What do you mean 'No'.  That is an opinion,  If you disagree, please
> explain why.

See above.

>>>> Changes like this:
>>>>> -        for (i=0;i<howmuch;i++)
>>>>> +        for (i=0;i<howmuch;++i)
>>>>>            TRACE("notify at %d to %p\n",
>>>>>                notify[i].dwOffset,notify[i].hEventNotify);
>>>> are not helpful at all.
>>> The post increment and decrement operation are specified as saving
>>> the original value for use in the evaluation of the expression they
>>> are part of and modifying the underlying stored value.  In expressions
>>> like this, that saved value is then discarded.  The optimization phase
>>> of the compilation usually removes both the save and discard operations.
>> Sure, but I don't think it's enough to justify such changes all over the
>> place, in existing code.
> I agree that it is not enough to justify a separate set of patches, but
> as part of another set of changes, I think it is justified.  After all,
> how else are examples of bad code going to be removed.

This is not a bad coding. You're changing a lot of my code, which uses
my coding style and this style is to use postfix incrementation. I'm not
saying it's better of worse because it's not, so there is no reason to
change it. Your argument about requirement for temporary storage is an
example of bad way to think about the code.


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