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Date:   Wed, 2 Oct 2019 15:21:36 +0300
From:   Mika Westerberg <mika.westerberg@linux.intel.com>
To:     Tejun Heo <tj@kernel.org>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        AceLan Kao <acelan.kao@canonical.com>,
        Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org
Subject: System hangs if NVMe/SSD is removed during suspend
Message-ID: <20191002122136.GD2819@lahna.fi.intel.com>
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Hi Tejun,

In a system with Thunderbolt connected NVMe or SSD entering system
suspend, detaching the NVMe/SSD and resuming the system hangs (see also
https://bugzilla.kernel.org/show_bug.cgi?id=204385).

Triggering sysrq-w I see this:

[  113.093783] Workqueue: nvme-wq nvme_remove_dead_ctrl_work [nvme]
[  113.095156] Call Trace:
[  113.096234]  ? __schedule+0x2c5/0x630
[  113.097409]  ? wait_for_completion+0xa4/0x120
[  113.098639]  schedule+0x3e/0xc0
[  113.099769]  schedule_timeout+0x1c9/0x320
[  113.100973]  ? resched_curr+0x1f/0xd0
[  113.102146]  ? wait_for_completion+0xa4/0x120
[  113.103379]  wait_for_completion+0xc3/0x120
[  113.104595]  ? wake_up_q+0x60/0x60
[  113.105749]  __flush_work+0x131/0x1e0
[  113.106925]  ? flush_workqueue_prep_pwqs+0x130/0x130
[  113.108215]  bdi_unregister+0xb9/0x130
[  113.109403]  del_gendisk+0x2d2/0x2e0
[  113.110580]  nvme_ns_remove+0xed/0x110 [nvme_core]
[  113.111853]  nvme_remove_namespaces+0x96/0xd0 [nvme_core]
[  113.113177]  nvme_remove+0x5b/0x160 [nvme]
[  113.114391]  pci_device_remove+0x36/0x90
[  113.115590]  device_release_driver_internal+0xdf/0x1c0
[  113.116893]  nvme_remove_dead_ctrl_work+0x14/0x30 [nvme]
[  113.118217]  process_one_work+0x1c2/0x3f0
[  113.119434]  worker_thread+0x48/0x3e0
[  113.120619]  kthread+0x100/0x140
[  113.121772]  ? current_work+0x30/0x30
[  113.122955]  ? kthread_park+0x80/0x80
[  113.124142]  ret_from_fork+0x35/0x40

The exact place is in wb_shutdown():

        /*
         * Drain work list and shutdown the delayed_work.  !WB_registered
         * tells wb_workfn() that @wb is dying and its work_list needs to
         * be drained no matter what.
         */
        mod_delayed_work(bdi_wq, &wb->dwork, 0);
        flush_delayed_work(&wb->dwork);

Now bdi_wq is marked as WQ_FREEZABLE and at this time we are still
resuming devices so I think it is still frozen. This basically results
that the resume process is waiting for bdi_unregister() but it cannot
progress because its workqueue is still frozen.

I saw you "dealt" similar situation for libata with commit 85fbd722ad0f
("libata, freezer: avoid block device removal while system is frozen")
and that there was discussion around this here:

  https://marc.info/?l=linux-kernel&m=138695698516487

but from that discussion I don't see more generic solution to be
implemented.

Any ideas we should fix this properly?

I mean nowadays Thunderbolt connected storage is quite common and I
suppose situation like this might wery well happen, say you have a dock
with SSD connected and you disconnect it when the laptop is suspended.

I've been using following hack to prevent the issue but I'm quite sure
there is a better solution ;-)

Thanks in advance!

 drivers/ata/libata-scsi.c | 2 ++
 mm/backing-dev.c          | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index 76d0f9de767b..3fea8d72f6f9 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -4791,6 +4791,7 @@ void ata_scsi_hotplug(struct work_struct *work)
 		return;
 	}
 
+#if 0
 	/*
 	 * XXX - UGLY HACK
 	 *
@@ -4810,6 +4811,7 @@ void ata_scsi_hotplug(struct work_struct *work)
 #ifdef CONFIG_FREEZER
 	while (pm_freezing)
 		msleep(10);
+#endif
 #endif
 
 	DPRINTK("ENTER\n");
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index e8e89158adec..8e77711d5dd0 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -236,8 +236,8 @@ static int __init default_bdi_init(void)
 {
 	int err;
 
-	bdi_wq = alloc_workqueue("writeback", WQ_MEM_RECLAIM | WQ_FREEZABLE |
-					      WQ_UNBOUND | WQ_SYSFS, 0);
+	bdi_wq = alloc_workqueue("writeback", WQ_MEM_RECLAIM | WQ_UNBOUND |
+					      WQ_SYSFS, 0);
 	if (!bdi_wq)
 		return -ENOMEM;