Re: [PATCH 2/2] target: iscsi: fix a race condition when aborting a task

[Michael Christie <michael.christie@oracle.com>]

> On Oct 27, 2020, at 12:54 PM, Mike Christie <michael.christie@oracle.com> wrote:
> 
> On 10/27/20 8:49 AM, Maurizio Lombardi wrote:
>> Hello Mike,
>> 
>> Dne 22. 10. 20 v 4:42 Mike Christie napsal(a):
>>> If we free the cmd from the abort path, then for your conn stop plus abort race case, could we do:
>>> 
>>> 1. thread1 runs iscsit_release_commands_from_conn and sets CMD_T_FABRIC_STOP.
>>> 2. thread2 runs iscsit_aborted_task and then does __iscsit_free_cmd. It then returns from the aborted_task callout and we finish target_handle_abort and do:
>>> 
>>> target_handle_abort -> transport_cmd_check_stop_to_fabric -> lio_check_stop_free -> target_put_sess_cmd
>>> 
>>> The cmd is now freed.
>>> 3. thread1 now finishes iscsit_release_commands_from_conn and runs iscsit_free_cmd while accessing a command we just released.
>>> 
>>> 
>> 
>> Thanks for the review!
>> 
>> There are definitely some problems with task aborts and commands' refcounting *
>> but this is a different bug than the one this patch is trying to solve (a race to list_del_init());
>> unless you are saying that abort tasks should never be executed when the connection 
>> is going down and we have to prevent such cases from happening at all.
> 
> Yeah, I think if we prevent the race then we fix the refcount issue and your issue.
> Here is a patch that is only compile tested:
> 
> From 209709bcedd9a6ce6003e6bb86f3ebf547dca6af Mon Sep 17 00:00:00 2001
> From: Mike Christie <michael.christie@oracle.com>
> Date: Tue, 27 Oct 2020 12:30:53 -0500
> Subject: [PATCH] iscsi target: fix cmd abort vs fabric stop race
> 
> The abort and cmd stop paths can race where:
> 
> 1. thread1 runs iscsit_release_commands_from_conn and sets
> CMD_T_FABRIC_STOP.
> 2. thread2 runs iscsit_aborted_task and then does __iscsit_free_cmd. It
> then returns from the aborted_task callout and we finish
> target_handle_abort and do:
> 
> target_handle_abort -> transport_cmd_check_stop_to_fabric ->
> lio_check_stop_free -> target_put_sess_cmd
> 
> The cmd is now freed.
> 3. thread1 now finishes iscsit_release_commands_from_conn and runs
> iscsit_free_cmd while accessing a command we just released.
> 
> In __target_check_io_state we check for CMD_T_FABRIC_STOP and set the
> CMD_T_ABORTED if the driver is not cleaning up the cmd because of
> a session shutdown. However, iscsit_release_commands_from_conn only
> sets the CMD_T_FABRIC_STOP and does not check to see if the abort path
> has claimed completion ownership of the command.
> 
> This adds a check in iscsit_release_commands_from_conn so only the
> abort or fabric stop path cleanup the command.
> ---
> drivers/target/iscsi/iscsi_target.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
> index f77e5ee..85027d3 100644
> --- a/drivers/target/iscsi/iscsi_target.c
> +++ b/drivers/target/iscsi/iscsi_target.c
> @@ -483,8 +483,7 @@ int iscsit_queue_rsp(struct iscsi_conn *conn, struct iscsi_cmd *cmd)
> void iscsit_aborted_task(struct iscsi_conn *conn, struct iscsi_cmd *cmd)
> {
> 	spin_lock_bh(&conn->cmd_lock);
> -	if (!list_empty(&cmd->i_conn_node) &&
> -	    !(cmd->se_cmd.transport_state & CMD_T_FABRIC_STOP))
> +	if (!list_empty(&cmd->i_conn_node))
> 		list_del_init(&cmd->i_conn_node);
> 	spin_unlock_bh(&conn->cmd_lock);
> 
> @@ -4088,6 +4087,16 @@ static void iscsit_release_commands_from_conn(struct iscsi_conn *conn)
> 
> 		if (se_cmd->se_tfo != NULL) {
> 			spin_lock_irq(&se_cmd->t_state_lock);
> +			if (se_cmd->transport_state & CMD_T_ABORTED) {
> +				/*
> +				 * LIO's abort path owns the cleanup for this,
> +				 * so put it back on the list and let
> +				 * aborted_task handle it.
> +				 */
> +				list_add_tail(&cmd->i_conn_node,
> +					      &conn->conn_cmd_list);


That should have been a move from the tmp list back to the conn_cmd_list.


> +				continue;
> +			}
> 			se_cmd->transport_state |= CMD_T_FABRIC_STOP;
> 			spin_unlock_irq(&se_cmd->t_state_lock);
> 		}
> -- 
> 1.8.3.1
>