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

[Julian Wiedmann]

On 21.05.21 20:56, Paul E. McKenney wrote:
> 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
> 

I expected no different ;). LGTM, and clearly emphasizing that one shall
not mix list_empty() with list_first_entry_rcu() is a nice improvement.


> ------------------------------------------------------------------------
> 
> 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.
>   */
>