Commits on Jul 11, 2018

1. Fix zpl_mount() deadlock …

   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>
   Issue openzfs#7691

   

   behlendorf committed Jul 11, 2018
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