Alan Stern <st...@rowland.harvard.edu> writes: > On Wed, 20 Feb 2013, Eric W. Biederman wrote: > >> >> Why can't the fuse filesystem freeze when there are requests pending? >> > >> > It _can_ freeze (that is, the fuse daemon can). The problem is that >> > tasks _using_ the fuse filsystem can't if the daemon doesn't respond. >> >> Which is what I meant when I said that the fuse filesystem couldn't >> freeze. > > Oh, okay. But it's no different from any other filesystem in that > respect. Processes generally can't be frozen while they are waiting > for filesystem I/O to complete. In many cases they can't receive > signals either (they are in an uninterruptible wait state).
Ick. So the process freezer and all network filesystems has problems? Especially nfs? >> > These tasks are stuck in uninterruptible wait states deep in the >> > filesystem layer, probably holding important locks. They can't be >> > frozen until the outstanding requests complete. >> >> Why is it that processes that can be preempted can't be frozen? > > There's a big difference between preemption and freezing: Preemption > is involuntary whereas freezing is voluntary. It's like the difference > between preemptive and cooperative multitasking. I hadn't realized freezing was voluntary. That certainly seems like a pain. > Processes can be frozen only by making explicit checks, and they > mustn't be frozen while they are holding locks that would prevent other > processes from reaching one of those checks. > >> At most I would suggest that processes be frozen in reverse priority >> order. Which unless there is a priority inversion should solve this >> problem without an additional proc file. > > Do fuse daemons (and the processes they rely upon) run with elevated > priority? I don't know if the daemons are of an elevated scheduling priority today but if they aren't it is as easy to require an elevated scheduling priority as it is to require a magic freezer priority. Furthermore if they don't run at an elevated priority there is the possibility of priority inversion. With a little care you might even be able to drop the kernel thread special case if you freeze processes by prirority. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
