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Synchronization and Multiprocessor Issues msdn documentation

Hi,

I've carrefullly read the Synchronization and Multiprocessor Issues msdn documentation and I'm wondering something about the "interlocked" solution to the "Fixing race condition" sample. From my understanding, Interlocked function are full memory barriers. It means that it both tells the compiler not to reorder the code around the barrier and the processor to flush cache lines (basically). So, why is the "iValue" accessed with an interlocked function? I mean, the "fValueHasBeenComputed" is used as a synchronization primitive and it is modified using interlocked functions which ensure the memory barrier functionnality. So I think "iValue" could be freelly accessed (ie without any interlock function) as long as synchronization is done with interlocked functions.

Am I wrong?

Many thanks
Guillaume
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In the referenced MSDN material I believe the sample code is poorly written. And lack of comments does it no good either.

Multiple processors can concurrently enter the ComputeValue() section of CacheComputedValue. This may be by design but it seems odd to me to have several processors concurrently executing ComputeValue().

I think in FetchComputedValue the

InterlockedExchange((LONG*)piResult, (LONG)iValue);

is not there to protect iValue but to protect the writing of piResult. A comment in the sample code might have cleared thing up.

Jim Dempsey

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Hi,

I agree some more comments would help.
My question was concerning the read write barriers introduced by interlock functions compared to mutexes. Could you confirm that using interlock functions to synchronize access to a variable can lead to the same level of correctness as using mutexes? I mean, if the synchronization's working, there's no need to manually call read and write barriers, right?

Regards
Guillaume
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Using mutex or spinlock or other event-like object is or may be recommended when an operation cannot be performed with a single Interlocked... operation. In reality the mutex, spinlock, ... now internally use Interlocked... operations in addition to the O/S calls that may occure if operation fails. Step into a SpinLock or Mutex and watch the code in the Disassembly window.

You can implement your own mutex operations using InterlockedExchange or other Interlocked... operations. One of the major reasons for using the O/S supplied routines is to reduce unnecessary processing overhead when a resource is in use and the thread owning the resource is suspended.

It used to be that to incriment a shared variable that you had to acquire a SpinLock which guards the variable, incriment the variable, (flush), then release the SpinLock. Interlocked instructions can now be used to accomplish the same without the use of SpinLock (or fast mutex).

In the case of the MSDN article they had a flag and a variable. Depending on the values of the variable a bit in the variable can be used as the flag. Then with the use of InterlockedCompareExchange you can contitionally update the variable and flag.

Do a web search on Lock Free Programming and LockFree/Wait Free Programming there are some good reference articles on this subject. MSDN does have some good reference material related to this but unfortunately it also has some bad material too.

The Interlocked instructions are processor dependent and have data size and alignment requirements. Also, they tend to be restricted to integer (32-bit or 64-bit) 128-bit is not commonly available. There are some interesting ways to perform a lock free add of floating point numbers through the use of local union of float and integer and whos integer part is conditionally swapped with InterlockedCompareExchange.

To answer your question, if you code your Interlocked... calls correctly then you do not need the mutexes, SpinLock, ...

Jim Dempsey

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