Fix /etc/hostid on root pool deadlock

Accidentally introduced by dc04a8c which now takes the SCL_VDEV lock
as a reader in zfs_blkptr_verify().  A deadlock can occur if the
/etc/hostid file resides on a dataset in the same pool.  This is
because reading the /etc/hostid file may occur while the caller is
holding the SCL_VDEV lock as a writer.  For example, to perform a
`zpool attach` as shown in the abbreviated stack below.

To resolve the issue we cache the system's hostid when initializing
the spa_t, or when modifying the multihost property.  The cached
value is then relied upon for subsequent accesses.

Call Trace:
    spa_config_enter+0x1e8/0x350 [zfs]
    zfs_blkptr_verify+0x33c/0x4f0 [zfs] <--- trying read lock
    zio_read+0x6c/0x140 [zfs]
    ...
    vfs_read+0xfc/0x1e0
    kernel_read+0x50/0x90
    ...
    spa_get_hostid+0x1c/0x38 [zfs]
    spa_config_generate+0x1a0/0x610 [zfs]
    vdev_label_init+0xa0/0xc80 [zfs]
    vdev_create+0x98/0xe0 [zfs]
    spa_vdev_attach+0x14c/0xb40 [zfs] <--- grabbed write lock

Reviewed-by: loli10K <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #9256 
Closes #9285 

1 files changed, 1 insertions, 1 deletions

diff --git a/module/zfs/spa_config.c b/module/zfs/spa_config.c
index 43da79dc3..de1ad21da 100644
--- a/module/zfs/spa_config.c
+++ b/module/zfs/spa_config.c
@@ -457,7 +457,7 @@ spa_config_generate(spa_t *spa, vdev_t *vd, uint64_t txg, int getstats)
 		fnvlist_add_string(config, ZPOOL_CONFIG_COMMENT,
 		    spa->spa_comment);
 
-	hostid = spa_get_hostid();
+	hostid = spa_get_hostid(spa);
 	if (hostid != 0)
 		fnvlist_add_uint64(config, ZPOOL_CONFIG_HOSTID, hostid);
 	fnvlist_add_string(config, ZPOOL_CONFIG_HOSTNAME, utsname()->nodename);