Reducing the cost of sinval messaging

I happened to be looking at sinvaladt.c and noticed the loop added in
commit b4fbe392f8ff6ff1a66b488eb7197eef9e1770a4:

/*
* Now that the maxMsgNum change is globally visible, we give everyone
* a swift kick to make sure they read the newly added messages.
* Releasing SInvalWriteLock will enforce a full memory barrier, so
* these (unlocked) changes will be committed to memory before we exit
* the function.
*/
for (i = 0; i < segP->lastBackend; i++)
{
ProcState *stateP = &segP->procState[i];

stateP->hasMessages = true;
}

This seems fairly awful when there are a large number of backends.

Why could we not remove the hasMessages flags again, and change the
unlocked test

if (!stateP->hasMessages)
return 0;

into

if (stateP->nextMsgNum == segP->maxMsgNum &&
!stateP->resetState)
return 0;

If we are looking at stale shared state, this test might produce a
false positive, but I don't see why it's any less safe than testing a
hasMessages boolean.

regards, tom lane