[Bug 167685] ZFS on USB drive prevents shutdown / reboot

From: <bugzilla-noreply_at_freebsd.org>
Date: Mon, 28 Aug 2023 13:30:11 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=167685

--- Comment #15 from John F. Carr <jfc@mit.edu> ---
If I run "zpool export sabrent" on the pool described in my previous comment
the command takes about 7 seconds to complete.

^T reports

load: 7.17  cmd: zpool 66858 [tx->tx_sync_done_cv] 4.07r 0.00u 2.54s 20% 8008k
mi_switch+0x152 sleepq_switch+0x104 _cv_wait+0x14a txg_wait_synced_impl+0xeb
txg_wait_synced+0xb zil_close+0x128 zfsvfs_teardown+0xb1 zfs_umount+0x129
dounmount+0x396 kern_unmount+0x312 amd64_syscall+0x13c fast_syscall_common+0xf8 
load: 6.76  cmd: zpool 66858 [running] 7.24r 0.00u 3.14s 20% 8024k
uma_zfree_arg+0xd6 abd_free_chunks+0x44 abd_free+0x45 arc_hdr_free_abd+0x144
arc_evict_state+0xaca arc_flush+0x8c dsl_pool_close+0xe3 spa_unload+0x349
spa_export_common+0x2c8 zfsdev_ioctl_common+0x5ce zfsdev_ioctl+0x12a
devfs_ioctl+0xd2 vn_ioctl+0x136 devfs_ioctl_f+0x1e kern_ioctl+0x286
sys_ioctl+0x140 amd64_syscall+0x13c fast_syscall_common+0xf8 

I see after filesystems are unmounted there is still some ARC cleanup to be
done.  The shutdown path has a call to vfs_unmountall in bufshutdown.  Does it
have a call out to ZFS to quiesce pools?

The hang is reproducible so I am able to test a fix if anybody has one.

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