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SubjectSystem hangs if NVMe/SSD is removed during suspend
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;
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