UBUNTU: SAUCE: (noup) zfs to 0.6.5.6-0ubuntu25

BugLink: http://bugs.launchpad.net/bugs/1781364

This is a sync of zfs 0.6.5.6-0ubuntu25 that fixes a mount/umount deadlock
for upstream ZFS commit ac09630d8b0b ("Fix zpl_mount() deadlock")

ZFS commit 93b43af inadvertently introduced the following scenario which
can result in a deadlock.  This issue was most easily reproduced by
LXD containers using a ZFS storage backend but should be reproducible
under any workload which is frequently mounting and unmounting.

-- THREAD A --
spa_sync()
  spa_sync_upgrades()
    rrw_enter(&dp->dp_config_rwlock, RW_WRITER, FTAG); <- Waiting on B

-- THREAD B --
mount_fs()
  zpl_mount()
    zpl_mount_impl()
      dmu_objset_hold()
        dmu_objset_hold_flags()
          dsl_pool_hold()
            dsl_pool_config_enter()
              rrw_enter(&dp->dp_config_rwlock, RW_READER, tag);
    sget()
      sget_userns()
        grab_super()
          down_write(&s->s_umount); <- Waiting on C

-- THREAD C --
cleanup_mnt()
  deactivate_super()
    down_write(&s->s_umount);
    deactivate_locked_super()
      zpl_kill_sb()
        kill_anon_super()
          generic_shutdown_super()
            sync_filesystem()
              zpl_sync_fs()
                zfs_sync()
                  zil_commit()
                    txg_wait_synced() <- Waiting on A
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
(backport from ZFS upstream commit ac09630d8b0bf6c92084a30fdaefd03fd0adbdc1)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

zfs/META

@@ -2,7 +2,7 @@ Meta:         1
 Name:         zfs
 Branch:       1.0
 Version:      0.6.5.6
-Release:      0ubuntu24
+Release:      0ubuntu25
 Release-Tags: relext
 License:      CDDL
 Author:       OpenZFS on Linux

zfs/include/sys/zfs_vfsops.h

@@ -32,6 +32,7 @@
 #include <sys/zil.h>
 #include <sys/sa.h>
 #include <sys/rrwlock.h>
+#include <sys/dsl_dataset.h>
 #include <sys/zfs_ioctl.h>
 
 #ifdef	__cplusplus

zfs/module/zfs/zpl_super.c

@@ -467,8 +467,17 @@ zpl_mount_impl(struct file_system_type *fs_type, int flags, zfs_mntopts_t *zmo)
 	if (err)
 		return (ERR_PTR(-err));
 
+	/*
+	 * The dsl pool lock must be released prior to calling sget().
+	 * It is possible sget() may block on the lock in grab_super()
+	 * while deactivate_super() holds that same lock and waits for
+	 * a txg sync.  If the dsl_pool lock is held over over sget()
+	 * this can prevent the pool sync and cause a deadlock.
+	 */
+	dsl_pool_rele(dmu_objset_pool(os), FTAG);
 	s = zpl_sget(fs_type, zpl_test_super, set_anon_super, flags, os);
-	dmu_objset_rele(os, FTAG);
+	dsl_dataset_rele(dmu_objset_ds(os), FTAG);
+
 	if (IS_ERR(s))
 		return (ERR_CAST(s));