Re: Code review request: small optimization to localtime.c

On Wed, 28 Nov 2007, M. Warner Losh wrote:

Please find enclosed some small optimizations. I know they make a
couple lines too long, I'll correct that before I commit. They make
the time functions do less redundant locking.

However, in many places we do the following:

pthread_mutex_lock();
if (!is_set) {
is_set = true;
// do something
}
pthread_mutex_unlock();

This is wasteful. We get locks ALL the time for every time operation,
when in fact we need it more rarely. If we can tolerate losing a
race, we can eliminate the locking in all but the startup case and
those threads racing the startup:

if (!is_set) {
pthread_mutex_lock();
if (!is_set) {
is_set = true;
// do something
}
pthread_mutex_unlock();
}

here, we know that is_set only ever changes from false to true. If it
is already true, there's nothing to do. If it is false, we may need
to do something, so we lock, check to see if we really need to do it,
etc.

Can anybody see a flaw in this logic?

Is this not a good place to use pthread_once() instead?

--
DE
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