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Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread_workqueue

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On Tue 02-08-16 10:06:12, Oliver Neukum wrote:
> On Mon, 2016-08-01 at 10:20 -0400, Tejun Heo wrote:
> > Hello,
> 
> > > If any real IO depends on those devices then this is not sufficient and
> > > they need some form of guarantee for progress (aka mempool).
> > 
> > Oliver, Alan, what do you think?  If USB itself can't operate without
> > allocating memory during transactions, whatever USB storage drivers
> 
> It cannot. The IO must be described to the hardware with a data
> structure in memory.
> 
> > are doing isn't all that meaningful.  Can we proceed with the
> > workqueue patches?  Also, it could be that the only thing GFP_NOIO and
> > GFP_ATOMIC are doing is increasing the chance of IO failures under
> > memory pressure.  Maybe it'd be a good idea to reconsider the
> > approach?
> 
> We had actual deadlocks with GFP_KERNEL. It seems to me that the SCSI
> layer can deal with IO that cannot be completed due to a lack of memory
> at least somewhat, but a deadlock within a driver would obviously be
> deadly. So I don't think that mempools would remove the need for
> GFP_NOIO as there are places in usbcore we cannot enter the page
> laundering path from. They are an additional need.

OK, I guess there is some misunderstanding here. I believe that Tejun
wasn't arguing to drop GFP_NOIO. It might be really needed for the dead
lock avoidance. No question about that. The whole point is that
WQ_RECLAIM might be completely pointless because a rescuer wouldn't help
much if the work item would do GFP_NOIO and get stuck in the page
allocator.
-- 
Michal Hocko
SUSE Labs

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[RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Bhaktipriya Shridhar <bhaktipriya96@gmail.com> - 2016-07-27 11:30 +0200
  Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Oliver Neukum <oneukum@suse.com> - 2016-07-27 11:40 +0200
    Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Tejun Heo <tj@kernel.org> - 2016-07-27 20:10 +0200
      Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread_workqueue Alan Stern <stern@rowland.harvard.edu> - 2016-07-27 21:00 +0200
        Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Tejun Heo <tj@kernel.org> - 2016-07-27 21:30 +0200
          Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread_workqueue Alan Stern <stern@rowland.harvard.edu> - 2016-07-27 22:50 +0200
            Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Tejun Heo <tj@kernel.org> - 2016-07-29 15:20 +0200
              Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Michal Hocko <mhocko@kernel.org> - 2016-08-01 16:00 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Tejun Heo <tj@kernel.org> - 2016-08-01 16:30 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated create_singlethread_workqueue Alan Stern <stern@rowland.harvard.edu> - 2016-08-01 21:10 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Michal Hocko <mhocko@kernel.org> - 2016-08-01 21:40 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Oliver Neukum <oneukum@suse.com> - 2016-08-02 10:20 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Michal Hocko <mhocko@kernel.org> - 2016-08-02 10:40 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Oliver Neukum <oneukum@suse.com> - 2016-08-02 12:10 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Michal Hocko <mhocko@kernel.org> - 2016-08-02 14:00 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Oliver Neukum <oneukum@suse.com> - 2016-08-02 15:40 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Tejun Heo <tj@kernel.org> - 2016-08-02 21:20 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Michal Hocko <mhocko@kernel.org> - 2016-08-02 23:40 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Michal Hocko <mhocko@kernel.org> - 2016-08-02 16:00 +0200
                Re: [RFC] usb: host: u132-hcd: Remove deprecated  create_singlethread_workqueue Oliver Neukum <oneukum@suse.com> - 2016-08-02 19:20 +0200

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