Scope of a Lock

Experience has shown that it is easier to deal with locks that are either held throughout the execution of a routine, or locks that are both acquired and released in one routine. Avoid nesting like this:

static void
 xxfoo(...)
 {
 	mutex_enter(&softc->lock);
 	...
 	xxbar();
 }
static void
 xxbar(...)
 {
 	...
 	mutex_exit(&softc->lock);
 }

This example works, but will almost certainly lead to maintenance problems.

If contention is likely in a particular code path, try to hold locks for a short time. In particular, arrange to drop locks before calling kernel routines that might block. For example:

mutex_enter(&softc->lock);
 			...
 softc->foo = bar;
 softc->thingp = kmem_alloc(sizeof(thing_t), KM_SLEEP);
 ...
 mutex_exit(&softc->lock);

This is better coded as:

thingp = kmem_alloc(sizeof(thing_t), KM_SLEEP);
 mutex_enter(&softc->lock);
 ...
 softc->foo = bar;
 softc->thingp = thingp;
 ...
 mutex_exit(&softc->lock);