Lock-free algorithms: Update if you can I'm feeling down

A customer was looking for advice on this synchronization problem:

We have a small amount of data that we need to share among multiple processes. One way to protect the data is to use a spin lock. However, that has potential for deadlock if the process which holds the spinlock doesn't get a chance to release it. For example, it might be suspended in the debugger, or somebody might decide to use Terminate­Process to nuke it.

Any suggestions on how we can share this data without exposure to these types of horrible failure modes? I'm thinking of something like a reader takes the lock, fetches the values, and then checks at status at the end of tell if the data is valid. Meanwhile, a writer tries to take the lock with a timeout, and if the timeout fires, then the writer just goes ahead anyway and updates the values, and somehow sets the status on the reader so it knows that the value is no good and it should try again. Basically, I don't want either the reader or writer to get stuck indefinitely if a developer, say, just happens to break into the debugger at the worst possible time.

This can be solved with a publishing pattern. When you want to update the values, you indicate that new values are ready by publishing their new location.

Let's say that the data that needs to be shared is a collection of four integers.

struct SHAREDDATA {

 int a, b, c, d;

};

Assume that there is a practical limit on how often the value can change; this is usually a safe assumption because you'll have some sort of external rate limiter, like "This value changes in response to a user action." (Even if there is no such limit, most solutions will simply posit one. For example, the SLIST functions simply assume that a processor won't get locked out more than 65535 times in a row.) In our case, let's say that the value will not change more than 64 times in rapid succession.

#define SHAREDDATA_MAXCONCURRENT 64

SHAREDDATA g_rgsd[SHAREDDATA_MAXCONCURRENT];

UINT g_isd; // current valid value

void GetSharedData(__out SHAREDDATA *psd)

{

 *psd = g_rgsd[g_isd];

}

Reading the data simply retrieves the most recently published value. The hard part is publishing the value.

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