A number of hangs have been reported against the target driver; they are
due to the fact that multiple threads may try to destroy the iscsi session
at the same time. This may be reproduced for example when a "targetcli
iscsi/iqn.../tpg1 disable" command is executed while a logout operation is
underway.

When this happens, two or more threads may end up sleeping and waiting for
iscsit_close_connection() to execute "complete(session_wait_comp)".  Only
one of the threads will wake up and proceed to destroy the session
structure, the remaining threads will hang forever.

Note that if the blocked threads are somehow forced to wake up with
complete_all(), they will try to free the same iscsi session structure
destroyed by the first thread, causing double frees, memory corruptions
etc...

With this patch, the threads that want to destroy the iscsi session will
increase the session refcount and will set the "session_close" flag to 1;
then they wait for the driver to close the remaining active connections.
When the last connection is closed, iscsit_close_connection() will wake up
all the threads and will wait for the session's refcount to reach zero;
when this happens, iscsit_close_connection() will destroy the session
structure because no one is referencing it anymore.

 INFO: task targetcli:5971 blocked for more than 120 seconds.
       Tainted: P           OE    4.15.0-72-generic #81~16.04.1
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 targetcli       D    0  5971      1 0x00000080
 Call Trace:
  __schedule+0x3d6/0x8b0
  ? vprintk_func+0x44/0xe0
  schedule+0x36/0x80
  schedule_timeout+0x1db/0x370
  ? __dynamic_pr_debug+0x8a/0xb0
  wait_for_completion+0xb4/0x140
  ? wake_up_q+0x70/0x70
  iscsit_free_session+0x13d/0x1a0 [iscsi_target_mod]
  iscsit_release_sessions_for_tpg+0x16b/0x1e0 [iscsi_target_mod]
  iscsit_tpg_disable_portal_group+0xca/0x1c0 [iscsi_target_mod]
  lio_target_tpg_enable_store+0x66/0xe0 [iscsi_target_mod]
  configfs_write_file+0xb9/0x120
  __vfs_write+0x1b/0x40
  vfs_write+0xb8/0x1b0
  SyS_write+0x5c/0xe0
  do_syscall_64+0x73/0x130
  entry_SYSCALL_64_after_hwframe+0x3d/0xa2

Link: https://lore.kernel.org/r/20200313170656.9716-3-mlombard@redhat.comReported-by: Matt Coleman <mcoleman@datto.com>
Tested-by: Matt Coleman <mcoleman@datto.com>
Tested-by: Rahul Kundu <rahul.kundu@chelsio.com>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>