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    SubjectRe: Freezable workqueue blocks non-freezable workqueue during the system resume process
    On Tue, Feb 23, 2016 at 10:34:09AM -0500, Alan Stern wrote:
    > On Tue, 23 Feb 2016, Peter Chen wrote:
    >
    > > Hi Tejun Heo and Florian Mickler,
    > >
    > > I have a question that during the system resume process, the freezable
    > > workqueue can be thawed if there is a non-freezable workqueue is
    > > blocked (At uninterruptable state)?
    > >
    > > My case like below, I have a USB OTG (Micro-AB) cable is at USB
    > > Micro-B port, and there is a USB driver on it, and un-plug this
    > > cable can wake up system from the suspend. There is a non-freezable
    > > workqueue ci_otg will be scheduled after disconnecting OTG cable,
    > > and in its worker ci_otg_work, it will try to disconnect USB drive,
    > > and flush disk information.
    >
    > These operations probably are not safe while the system is resuming.
    > It might be best to make them wait until the resume is finished.
    >
    > > But flush disk information is done by
    > > freezable workqueue writeback, it seeems workqueue writeback is
    > > never got chance to execute, the workqueue ci_otg is waiting there
    > > forever, and the system is deadlock.
    >
    > > Both change workqueue ci_otg as freezable or change workqueue writeback
    > > as non-freezable can fix this problem.
    >
    > It sounds like making ci_otg freezable is the easiest solution.
    >
    > > Please ignore it, the system is locked at driver's resume,
    > > maybe at scsi or usb driver, so of cos, the freezable processes
    > > can't be thawed.
    >
    > > > [ 555.263177] [<c0043b1c>] (flush_work) from [<c0043fac>] (flush_delayed_work+0x48/0x4c)
    > > > [ 555.271106] r8:ed5b5000 r7:c0b38a3c r6:eea439cc r5:eea4372c r4:eea4372c
    > > > [ 555.277958] [<c0043f64>] (flush_delayed_work) from [<c00eae18>] (bdi_unregister+0x84/0xec)
    > > > [ 555.286236] r4:eea43520 r3:20000153
    > > > [ 555.289885] [<c00ead94>] (bdi_unregister) from [<c02c2154>] (blk_cleanup_queue+0x180/0x29c)
    > > > [ 555.298250] r5:eea43808 r4:eea43400
    >
    > You might want to complain to the block-layer people about this. I
    > don't know if anything can be done to fix it.
    >
    > Or maybe flush_work and flush_delayed_work can be changed to avoid
    > blocking if the workqueue is frozen. Tejun?
    >

    I have a patch to show the root cause of this issue.

    http://www.spinics.net/lists/linux-usb/msg136815.html

    --

    Best Regards,
    Peter Chen

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