target: fix race during implicit transition work flushes

This fixes the following races:

1. core_alua_do_transition_tg_pt could have read
tg_pt_gp_alua_access_state and gone into this if chunk:

if (!explicit &&
        atomic_read(&tg_pt_gp->tg_pt_gp_alua_access_state) ==
           ALUA_ACCESS_STATE_TRANSITION) {

and then core_alua_do_transition_tg_pt_work could update the
state. core_alua_do_transition_tg_pt would then only set
tg_pt_gp_alua_pending_state and the tg_pt_gp_alua_access_state would
not get updated with the second calls state.

2. core_alua_do_transition_tg_pt could be setting
tg_pt_gp_transition_complete while the tg_pt_gp_transition_work
is already completing. core_alua_do_transition_tg_pt then waits on the
completion that will never be called.

To handle these issues, we just call flush_work which will return when
core_alua_do_transition_tg_pt_work has completed so there is no need
to do the complete/wait. And, if core_alua_do_transition_tg_pt_work
was running, instead of trying to sneak in the state change, we just
schedule up another core_alua_do_transition_tg_pt_work call.

Note that this does not handle a possible race where there are multiple
threads call core_alua_do_transition_tg_pt at the same time. I think
we need a mutex in target_tg_pt_gp_alua_access_state_store.
Signed-off-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_alua.c

@@ -1079,16 +1079,8 @@ static int core_alua_do_transition_tg_pt(
 	/*
 	 * Flush any pending transitions
 	 */
-	if (!explicit && atomic_read(&tg_pt_gp->tg_pt_gp_alua_access_state) ==
-	    ALUA_ACCESS_STATE_TRANSITION) {
-		/* Just in case */
-		tg_pt_gp->tg_pt_gp_alua_pending_state = new_state;
-		tg_pt_gp->tg_pt_gp_transition_complete = &wait;
+	if (!explicit)
 		flush_work(&tg_pt_gp->tg_pt_gp_transition_work);
-		wait_for_completion(&wait);
-		tg_pt_gp->tg_pt_gp_transition_complete = NULL;
-		return 0;
-	}
 
 	/*
 	 * Save the old primary ALUA access state, and set the current state