Fix PSCI CPU ON race when setting state to ON_PENDING

When a CPU is powered down using PSCI CPU OFF API, it disables its caches
and updates its `aff_info_state` to OFF. The corresponding cache line is
invalidated by the CPU so that the update will be observed by other CPUs
running with caches enabled. There is a possibility that another CPU
which has been trying to turn ON this CPU via PSCI CPU ON API,
has already seen the update to `aff_info_state` and proceeds to update
the state to ON_PENDING prior to the cache invalidation. This may result
in the update of the state to ON_PENDING being discarded.

This patch fixes this issue by making sure that the update of `aff_info_state`
to ON_PENDING sticks by reading back the value after the cache flush and
retrying it if not updated. The patch also adds a dsbish() to
`psci_do_cpu_off()` to ensure ordering of the update to `aff_info_state`
prior to cache line invalidation.

Fixes ARM-software/tf-issues#349

Change-Id: I225de99957fe89871f8c57bcfc243956e805dcca

services/std_svc/psci/psci_off.c

@@ -129,10 +129,13 @@ exit:
 		 * Set the affinity info state to OFF. This writes directly to
 		 * main memory as caches are disabled, so cache maintenance is
 		 * required to ensure that later cached reads of aff_info_state
-		 * return AFF_STATE_OFF.
+		 * return AFF_STATE_OFF.  A dsbish() ensures ordering of the
+		 * update to the affinity info state prior to cache line
+		 * invalidation.
 		 */
 		flush_cpu_data(psci_svc_cpu_data.aff_info_state);
 		psci_set_aff_info_state(AFF_STATE_OFF);
+		dsbish();
 		inv_cpu_data(psci_svc_cpu_data.aff_info_state);
 
 		/*

services/std_svc/psci/psci_on.c

@@ -56,24 +56,6 @@ static int cpu_on_validate_state(aff_info_state_t aff_state)
 	return PSCI_E_SUCCESS;
 }
 
-/*******************************************************************************
- * This function sets the aff_info_state in the per-cpu data of the CPU
- * specified by cpu_idx
- ******************************************************************************/
-static void psci_set_aff_info_state_by_idx(unsigned int cpu_idx,
-					   aff_info_state_t aff_state)
-{
-
-	set_cpu_data_by_index(cpu_idx,
-			      psci_svc_cpu_data.aff_info_state,
-			      aff_state);
-
-	/*
-	 * Flush aff_info_state as it will be accessed with caches turned OFF.
-	 */
-	flush_cpu_data_by_index(cpu_idx, psci_svc_cpu_data.aff_info_state);
-}
-
 /*******************************************************************************
  * Generic handler which is called to physically power on a cpu identified by
  * its mpidr. It performs the generic, architectural, platform setup and state
@@ -90,6 +72,7 @@ int psci_cpu_on_start(u_register_t target_cpu,
 {
 	int rc;
 	unsigned int target_idx = plat_core_pos_by_mpidr(target_cpu);
+	aff_info_state_t target_aff_state;
 
 	/*
 	 * This function must only be called on platforms where the
@@ -119,8 +102,26 @@ int psci_cpu_on_start(u_register_t target_cpu,
 
 	/*
 	 * Set the Affinity info state of the target cpu to ON_PENDING.
+	 * Flush aff_info_state as it will be accessed with caches
+	 * turned OFF.
+	 */
+	psci_set_aff_info_state_by_idx(target_idx, AFF_STATE_ON_PENDING);
+	flush_cpu_data_by_index(target_idx, psci_svc_cpu_data.aff_info_state);
+
+	/*
+	 * The cache line invalidation by the target CPU after setting the
+	 * state to OFF (see psci_do_cpu_off()), could cause the update to
+	 * aff_info_state to be invalidated. Retry the update if the target
+	 * CPU aff_info_state is not ON_PENDING.
 	 */
+	target_aff_state = psci_get_aff_info_state_by_idx(target_idx);
+	if (target_aff_state != AFF_STATE_ON_PENDING) {
+		assert(target_aff_state == AFF_STATE_OFF);
 		psci_set_aff_info_state_by_idx(target_idx, AFF_STATE_ON_PENDING);
+		flush_cpu_data_by_index(target_idx, psci_svc_cpu_data.aff_info_state);
+
+		assert(psci_get_aff_info_state_by_idx(target_idx) == AFF_STATE_ON_PENDING);
+	}
 
 	/*
 	 * Perform generic, architecture and platform specific handling.
@@ -136,9 +137,11 @@ int psci_cpu_on_start(u_register_t target_cpu,
 	if (rc == PSCI_E_SUCCESS)
 		/* Store the re-entry information for the non-secure world. */
 		cm_init_context_by_index(target_idx, ep);
-	else
+	else {
 		/* Restore the state on error. */
 		psci_set_aff_info_state_by_idx(target_idx, AFF_STATE_OFF);
+		flush_cpu_data_by_index(target_idx, psci_svc_cpu_data.aff_info_state);
+	}
 
 exit:
 	psci_spin_unlock_cpu(target_idx);

services/std_svc/psci/psci_private.h

@@ -78,6 +78,9 @@
 		get_cpu_data(psci_svc_cpu_data.aff_info_state)
 #define psci_get_aff_info_state_by_idx(idx) \
 		get_cpu_data_by_index(idx, psci_svc_cpu_data.aff_info_state)
+#define psci_set_aff_info_state_by_idx(idx, aff_state) \
+		set_cpu_data_by_index(idx, psci_svc_cpu_data.aff_info_state,\
+					aff_state)
 #define psci_get_suspend_pwrlvl() \
 		get_cpu_data(psci_svc_cpu_data.target_pwrlvl)
 #define psci_set_suspend_pwrlvl(target_lvl) \