RE: [ros-dev] Re: [ros-diffs] [gedmurphy] 19312: Improve stopping control of the service

[Murphy, Ged (Bolton)]

> -----Original Message-----
> From: Thomas Weidenmueller [mailto:[e-mail]] 
> Sent: 17 November 2005 23:38
> To: ReactOS Development List
> Subject: Re: [ros-dev] Re: [ros-diffs] [gedmurphy] 19312: 
> Improve stopping control of the service
>
> Ged Murphy wrote:
> > Could you explain what you mean?
>
> As you want to change the value of the variables which are used in a
> multithreaded environment you need to used the Interlocked* 
> functions to
> ensure an atomic change in all environments (UP, MP, ...), and also to
> make sure it's portable.
> > Those variables are only modified within the base thread, all other
> > threads just check the value and act upon it.
> > Why would that not be thread safe?
>
> You're assuming the code will only run on a x86 system. In 
> fact, as long
> as the variables are aligned properly it is an atomic operation. But
> don't forget the other architectures and possibly upcoming 
> technologies
> based on x86 that could void this assumption.
>
> I have to admit that the code wouldn't break, even in circumstances
> where the operation wouldn't be atomic, but technically the code still
> isn't thread-safe this way.
> Thomas

This function is new to me. 
I'm just doing a bit of research on it now to better understand it, but
could you just give me a bit of background of the difference between this,
and using something like a mutex or CS, and why it's better in this
scenario?

Thanks,
Ged.

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[Thomas Weidenmueller]

Murphy, Ged (Bolton) wrote:
> This function is new to me.

Now that you're writing multithreaded code it's about time to learn
about these functions ;)
> I'm just doing a bit of research on it now to better understand it, but
> could you just give me a bit of background of the difference between this,
> and using something like a mutex or CS, and why it's better in this
> scenario?

Of course you can use mutexes or critical sections, but for this purpose
interlocked operations are much more lightweight because with mutexes or
critical sections you'd have to protect the read operation as well. The
Interlocked* functions basically guarantee that a certain operation on a
variable shared by multiple threads is performed in an atomic matter. I
probably should mention that reads are always atomic, so reading a
variable with the size smaller or equal as the size of a general purpose
register does not need to be protected, writes however have to be protected.

You should read the following articles:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dllproc/bas...
http://blogs.msdn.com/oldnewthing/archive/2004/09/15/229915.aspx