iscsi-target: Fix immediate queue starvation regression with DATAIN

This patch addresses a v3.5+ regression in iscsi-target where TX thread
process context -> handle_response_queue() execution is allowed to run
unbounded while servicing constant outgoing flow of ISTATE_SEND_DATAIN
response state.

This ends up preventing memory release of StatSN acknowledged commands
in a timely manner when under heavy large block streaming DATAIN
workloads.

The regression bug was initially introduced with:

commit 6f3c0e69a9c20441bdc6d3b2d18b83b244384ec6
Author: Andy Grover <agrover@redhat.com>
Date:   Tue Apr 3 15:51:09 2012 -0700

    target/iscsi: Refactor target_tx_thread immediate+response queue loops

Go ahead and follow original iscsi_target_tx_thread() logic and check
to break for immediate queue processing after each DataIN Sequence and/or
Response PDU has been sent.

Reported-by: Benjamin ESTRABAUD <be@mpstor.com>
Cc: Andy Grover <agrover@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
index 23a98e658306..af77396234a2 100644
--- a/drivers/target/iscsi/iscsi_target.c
+++ b/drivers/target/iscsi/iscsi_target.c
@@ -3583,6 +3583,10 @@ check_rsp_state:
                                 spin_lock_bh(&cmd->istate_lock);
                                 cmd->i_state = ISTATE_SENT_STATUS;
                                 spin_unlock_bh(&cmd->istate_lock);
+
+                                if (atomic_read(&conn->check_immediate_queue))
+                                        return 1;
+
                                 continue;
                         } else if (ret == 2) {
                                 /* Still must send status,
@@ -3672,7 +3676,7 @@ check_rsp_state:
                 }
 
                 if (atomic_read(&conn->check_immediate_queue))
-                        break;
+                        return 1;
         }
 
         return 0;
@@ -3716,12 +3720,15 @@ restart:
                      signal_pending(current))
                         goto transport_err;
 
+get_immediate:
                 ret = handle_immediate_queue(conn);
                 if (ret < 0)
                         goto transport_err;
 
                 ret = handle_response_queue(conn);
-                if (ret == -EAGAIN)
+                if (ret == 1)
+                        goto get_immediate;
+                else if (ret == -EAGAIN)
                         goto restart;
                 else if (ret < 0)
                         goto transport_err;