Re: [ANNOUNCE] Kernel Janitor's TODO list

Andi Kleen
2001-01-29T17:26:59Z

On Mon, Jan 29, 2001 at 11:01:31AM -0600, Timur Tabi wrote:
> What makes it more frustrating is that some people on this list talk as if
> things things are common knowledge. I've been following this mailing list for
> months, and until today I had no idea sleep_on was bad. All the documentation
> I've read to date freely uses sleep_on in the sample code. In fact, I still

When Linux documentation uses sleep_on it is probably broken and should be
fixed. Unix (not linux) documentation uses sleep_on commonly, but Unix has
different wait queue semantics and it is usually safe there.

You're probably reading the wrong documentation, e.g. Rusty's
kernel hacking HOWTO describes it correctly (and a lot of the other rules)

> don't even know WHY it's bad. Not only that, but what am I supposed to use
> instead?

You can miss wakeups. The standard pattern is:

get locks

add_wait_queue(&waitqueue, &wait);
for (;;) {
if (condition you're waiting for is true)
break;
unlock any non sleeping locks you need for condition
__set_task_state(current, TASK_UNINTERRUPTIBLE);
schedule();
__set_task_state(current, TASK_RUNNING);
reaquire locks
}
remove_wait_queue(&waitqueue, &wait);

When you want to handle signals you can check for them before or after the
condition check. Also use TASK_INTERRUPTIBLE in this case.

When you need a timeout use schedule_timeout().

For some cases you can also use the wait_event_* macros which encapsulate
that for you, assuming condition can be tested/used lockless.

An alternative is to use a semaphore, although that behaves a bit differently
under load.

> This is what I find most frustrating about Linux. If I were a Windows driver
> programmer, I could walk into any bookstore and pick up any of a dozen books
> that explains everything, leaving no room for doubt.

Just why are Windows drivers so buggy then?

-Andi