Messages in this thread | | | Date | Tue, 6 Mar 2012 17:14:16 +0400 | From | Glauber Costa <> | Subject | Re: [RFC PATCH] checkpatch: Warn on use of yield() |
| |
On 03/06/2012 04:45 PM, Peter Zijlstra wrote: > 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. >
Can't we point people to some Documentation file that explains the alternatives?
| |