rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.

The build will break if you change the Kconfig to allow DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning near where the breakage would occur. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT.
The build will break if you change the Kconfig to allow DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning near where the breakage would occur. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Paul E. McKenney
1 parent fe8e64071a
Showing 1 changed file with 5 additions and 0 deletions Side-by-side Diff
kernel/rcupdate.c
@@ -214,6 +214,11 @@
 		 * Ensure that queued callbacks are all executed.
 		 * If we detect that we are nested in a RCU read-side critical
 		 * section, we should simply fail, otherwise we would deadlock.
+		 * Note that the machinery to reliably determine whether
+		 * or not we are in an RCU read-side critical section
+		 * exists only in the preemptible RCU implementations
+		 * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
+		 * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
 		 */
 		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
 		    irqs_disabled()) {
...	...	@@ -214,6 +214,11 @@
214	214	* Ensure that queued callbacks are all executed.
215	215	* If we detect that we are nested in a RCU read-side critical
216	216	* section, we should simply fail, otherwise we would deadlock.
	217	+ * Note that the machinery to reliably determine whether
	218	+ * or not we are in an RCU read-side critical section
	219	+ * exists only in the preemptible RCU implementations
	220	+ * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why
	221	+ * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT.
217	222	*/
218	223	if (rcu_preempt_depth() != 0 \|\| preempt_count() != 0 \|\|
219	224	irqs_disabled()) {