On Mon, 14 Mar 2016, Jan Kara wrote: > On Fri 11-03-16 12:56:10, Tejun Heo wrote: > > Hello, Jan. > > > > On Thu, Mar 03, 2016 at 10:33:10AM +0100, Jan Kara wrote: > > > > Ugh... that's nasty. I wonder whether the right thing to do is making > > > > writeback workers non-freezable. IOs are supposed to be blocked from > > > > lower layer anyway. Jan, what do you think? > > > > > > Well no, at least currently IO is not blocked in lower layers AFAIK - for > > > that you'd need to freeze block devices & filesystems and there are issues > > > > At least libata does and I think SCSI does too, but yeah, there > > probably are drivers which depend on block layer blocking IOs, which > > btw is a pretty fragile way to go about as upper layers might not be > > the only source of activities. > > > > > with that (Jiri Kosina was the last one which was trying to make this work > > > AFAIR). And I think you need to stop writeback (and generally any IO) to be > > > generated so that it doesn't interact in a strange way with device drivers > > > being frozen. So IMO until suspend freezes filesystems & devices properly > > > you have to freeze writeback workqueue. What do you mean by "freezes ... devices"? Only a piece of code can be frozen -- not a device. The kernel does suspend device drivers; that is, it invokes their suspend callbacks. But it doesn't "freeze" them in any sense. Once a driver has been suspended, it assumes it won't receive any I/O requests until it has been resumed. Therefore the kernel first has to prevent all the upper layers from generating such requests and/or sending them to the low-level drivers. > > I still think the right thing to do is plugging that block layer or > > low level drivers. It's like we're trying to plug multiple sources > > when we can plug the point where they come together anyway. > > I agree that freezing writeback workers is a workaround for real issues at > best and ideally we shouldn't have to do that. But at least for now I had > the impression that it is needed for suspend to work reasonably reliably. The design is not to plug low-level drivers, but instead to prevent them from receiving any requests by plugging or freezing high-level code. It's pretty clear that we don't want to have ongoing I/O during a system suspend, right? And that means the I/O has to be prevented (or "plugged", if you prefer) somewhere -- either at an upper layer or at a lower layer. There was a choice to be made, and the decision was to do it at an upper layer. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
