On Mon, 2012-03-05 at 18:01 -0800, Joe Perches wrote:
> +# check for use of yield()
> + if ($line =~ /\byield\s*\(\s*\)/ {
> + WARN("YIELD",
> + "yield() is deprecated, consider cpu_relax()\n" . $herecurr);
> + }
Its not deprecated as such, its just a very dangerous and ill considered
API.
cpu_relax() is not a good substitute suggestion in that its still a busy
wait and prone to much of the same problems.
The case at hand was a life-lock due to expecting that yield() would run
another process which it needed in order to complete. Yield() does not
provide that guarantee.
Looking at fs/ext4/mballoc.c, we have this gem:
/*
* Yield the CPU here so that we don't get soft lockup
* in non preempt case.
*/
yield();
This is of course complete crap as well.. I suspect they want
cond_resched() there. And:
/* let others to free the space */
yield();
Like said, yield() doesn't guarantee anything like running anybody else,
does it rely on that? Or is it optimistic?
Another fun user:
void tasklet_kill(struct tasklet_struct *t)
{
if (in_interrupt())
printk("Attempt to kill tasklet from interrupt\n");
while (test_and_set_bit(TASKLET_STATE_SCHED, &t->state)) {
do {
yield();
} while (test_bit(TASKLET_STATE_SCHED, &t->state));
}
tasklet_unlock_wait(t);
clear_bit(TASKLET_STATE_SCHED, &t->state);
}
The only reason that doesn't explode is because running tasklets is
non-preemptible, However since they're non-preemptible they shouldn't
run long and you might as well busy spin. If they can run long, yield()
isn't your biggest problem.
mm/memory_hotplug.c has two yield() calls in offline_pages() and I've no
idea what they're trying to achieve.
But really, yield() is basically _always_ the wrong thing. The right
thing can be:
cond_resched(); wait_event(); or something entirely different.
So instead of suggesting an alternative, I would suggest thinking about
the actual problem in order to avoid the non-thinking solutions the
checkpatch brigade is so overly fond of :/
Maybe something like:
"yield() is dangerous and wrong, rework your code to not use it."
That at least requires some sort of thinking and doesn't suggest blind
substitution.
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