[PATCH RFC 2/8] rcu: Clarify usage of cond_resched for tasks-RCU

[Joel Fernandes (Google)]

Recently we had a discussion about cond_resched unconditionally
recording a voluntary context switch [1].

Lets add a comment clarifying that how this API is to be used.

[1] https://lkml.kernel.org/r/1526027434-21237-1-git-send-email-byungchul.park@xxxxxxx

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 include/linux/rcupdate.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 743226176350..a9881007ece6 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -159,8 +159,12 @@ static inline void rcu_init_nohz(void) { }
 	} while (0)
 
 /*
- * Note a voluntary context switch for RCU-tasks benefit.  This is a
- * macro rather than an inline function to avoid #include hell.
+ * Note an attempt to perform a voluntary context switch for RCU-tasks benefit.
+ *
+ * This is called even in situations where a context switch didn't really
+ * happen even though it was requested. The caller uses it to indicate
+ * traversal of an RCU-tasks quiescent state. This is a macro rather than an
+ * inline function to avoid #include hell.
  */
 #ifdef CONFIG_TASKS_RCU
 #define rcu_note_voluntary_context_switch_lite(t) \
@@ -187,7 +191,8 @@ static inline void exit_tasks_rcu_finish(void) { }
 #endif /* #else #ifdef CONFIG_TASKS_RCU */
 
 /**
- * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
+ * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU.
+ * The quiescent state report is made even if cond_resched() did nothing.
  *
  * This macro resembles cond_resched(), except that it is defined to
  * report potential quiescent states to RCU-tasks even if the cond_resched()
-- 
2.17.0.441.gb46fe60e1d-goog