Re: [PATCH] rculist: unify documentation about missing list_empty_rcu()

[Paul E. McKenney]

On Fri, May 21, 2021 at 12:08:29PM +0200, Julian Wiedmann wrote:
> We have two separate sections that talk about why list_empty_rcu()
> is not needed, consolidate them.
> 
> Signed-off-by: Julian Wiedmann <jwi@xxxxxxxxxxxxx>

Good catch, thank you!  As usual, I could not resist the urge to further
wordsmith, resulting in the following.  Please let me know if I messed
anything up.

							Thanx, Paul

------------------------------------------------------------------------

commit 6e9da58a4b391035e1ce77b8d867cdcdc73521b2
Author: Julian Wiedmann <jwi@xxxxxxxxxxxxx>
Date:   Fri May 21 12:08:29 2021 +0200

    rculist: Unify documentation about missing list_empty_rcu()
    
    We have two separate sections that talk about why list_empty_rcu()
    is not needed, so this commit consolidates them.
    
    Signed-off-by: Julian Wiedmann <jwi@xxxxxxxxxxxxx>
    [ paulmck: The usual wordsmithing. ]
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index f8633d37e358..d29740be4833 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -10,15 +10,6 @@
 #include <linux/list.h>
 #include <linux/rcupdate.h>
 
-/*
- * Why is there no list_empty_rcu()?  Because list_empty() serves this
- * purpose.  The list_empty() function fetches the RCU-protected pointer
- * and compares it to the address of the list head, but neither dereferences
- * this pointer itself nor provides this pointer to the caller.  Therefore,
- * it is not necessary to use rcu_dereference(), so that list_empty() can
- * be used anywhere you would want to use a list_empty_rcu().
- */
-
 /*
  * INIT_LIST_HEAD_RCU - Initialize a list_head visible to RCU readers
  * @list: list to be initialized
@@ -318,21 +309,29 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
 /*
  * Where are list_empty_rcu() and list_first_entry_rcu()?
  *
- * Implementing those functions following their counterparts list_empty() and
- * list_first_entry() is not advisable because they lead to subtle race
- * conditions as the following snippet shows:
+ * They do not exist because they would lead to subtle race conditions:
  *
  * if (!list_empty_rcu(mylist)) {
  *	struct foo *bar = list_first_entry_rcu(mylist, struct foo, list_member);
  *	do_something(bar);
  * }
  *
- * The list may not be empty when list_empty_rcu checks it, but it may be when
- * list_first_entry_rcu rereads the ->next pointer.
- *
- * Rereading the ->next pointer is not a problem for list_empty() and
- * list_first_entry() because they would be protected by a lock that blocks
- * writers.
+ * The list might be non-empty when list_empty_rcu() checks it, but it
+ * might have become empty by the time that list_first_entry_rcu() rereads
+ * the ->next pointer, which would result in a SEGV.
+ *
+ * When not using RCU, it is OK for list_first_entry() to re-read that
+ * pointer because both functions should be protected by some lock that
+ * blocks writers.
+ *
+ * When using RCU, list_empty() uses READ_ONCE() to fetch the
+ * RCU-protected ->next pointer and then compares it to the address of the
+ * list head.  However, it neither dereferences this pointer nor provides
+ * this pointer to its caller.  Thus, READ_ONCE() suffices (that is,
+ * rcu_dereference() is not needed), which means that list_empty() can be
+ * used anywhere you would want to use list_empty_rcu().  Just don't
+ * expect anything useful to happen if you do a subsequent lockless
+ * call to list_first_entry_rcu()!!!
  *
  * See list_first_or_null_rcu for an alternative.
  */