Re: [PATCH] fs-writeback: drop wb->list_lock during blk_finish_plug()

From: Linus Torvalds
Date: Mon Sep 28 2015 - 12:08:24 EST

On Mon, Sep 28, 2015 at 10:47 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> It gets set by preemption - and,
>> somewhat illogically, by cond_resched().
> I suspect that was done to make cond_resched() (voluntary preemption)
> more robust and only have a single preemption path/logic. But all that
> was done well before I got involved.

So I think it's actually the name that is bad, not necessarily the behavior.

We tend to put "cond_resched()" (and particularly
"cond_resched_lock()") in some fairly awkward places, and it's not
always entirely clear that task->state == TASK_RUNNING there.

So the preemptive behavior of not *really* putting the task to sleep
may actually be the right one. But it is rather non-intuitive given
the name - because "cond_resched()" basically is not at all equivalent
to "if (need_resched()) schedule()", which you'd kind of expect.

An explicit schedule will actually act on the task->state, and make us
go to sleep. "cond_resched()" really is just a "voluntary preemption
point". And I think it would be better if it got named that way.

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