Date:   Tue, 31 Jul 2018 15:04:48 +1000
From:   NeilBrown <neilb@...e.com>
To:     paulmck@...ux.vnet.ibm.com
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        Thomas Graf <tgraf@...g.ch>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] rhashtable: don't hold lock on first table throughout insertion.

On Mon, Jul 30 2018, Paul E. McKenney wrote:

> On Tue, Jul 31, 2018 at 10:45:45AM +1000, NeilBrown wrote:
>> On Fri, Jul 27 2018, Paul E. McKenney wrote:
>> 
>> > On Thu, Jul 26, 2018 at 08:18:15PM -0700, Paul E. McKenney wrote:
>> >> On Fri, Jul 27, 2018 at 11:04:37AM +1000, NeilBrown wrote:
>> >> > On Wed, Jul 25 2018, Paul E. McKenney wrote:
>> >> > >> 
>> >> > >> Looks good ... except ... naming is hard.
>> >> > >> 
>> >> > >>  is_after_call_rcu_init()  asserts where in the lifecycle we are,
>> >> > >>  is_after_call_rcu() tests where in the lifecycle we are.
>> >> > >> 
>> >> > >>  The names are similar but the purpose is quite different.
>> >> > >>  Maybe s/is_after_call_rcu_init/call_rcu_init/ ??
>> >> > >
>> >> > > How about rcu_head_init() and rcu_head_after_call_rcu()?
>> >> 
>> >> Very well, I will pull this change in on my next rebase.
>> >
>> > Like this?
>> 
>> Hard to say - unwinding white-space damage in my head is too challenging
>> when newlines have been deleted :-(
>
> What???  Don't you like block-structured code?
>
> All kidding aside, how about the following more conventionally formatted
> version?

Wow - it's like I just got new glasses!
Yes - nice an clear and now flaws to be found.  Thanks a lot.

NeilBrown

>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit e3408141ed7d702995b2fdc94703af88aadd226b
> Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Date:   Tue Jul 24 15:28:09 2018 -0700
>
>     rcu: Provide functions for determining if call_rcu() has been invoked
>     
>     This commit adds rcu_head_init() and rcu_head_after_call_rcu() functions
>     to help RCU users detect when another CPU has passed the specified
>     rcu_head structure and function to call_rcu().  The rcu_head_init()
>     should be invoked before making the structure visible to RCU readers,
>     and then the rcu_head_after_call_rcu() may be invoked from within
>     an RCU read-side critical section on an rcu_head structure that
>     was obtained during a traversal of the data structure in question.
>     The rcu_head_after_call_rcu() function will return true if the rcu_head
>     structure has already been passed (with the specified function) to
>     call_rcu(), otherwise it will return false.
>     
>     If rcu_head_init() has not been invoked on the rcu_head structure
>     or if the rcu_head (AKA callback) has already been invoked, then
>     rcu_head_after_call_rcu() will do WARN_ON_ONCE().
>     
>     Reported-by: NeilBrown <neilb@...e.com>
>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>     [ paulmck: Apply neilb naming feedback. ]
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index e4f821165d0b..4db8bcacc51a 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -857,6 +857,46 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
>  #endif /* #else #ifdef CONFIG_ARCH_WEAK_RELEASE_ACQUIRE */
>  
>  
> +/* Has the specified rcu_head structure been handed to call_rcu()? */
> +
> +/*
> + * rcu_head_init - Initialize rcu_head for rcu_head_after_call_rcu()
> + * @rhp: The rcu_head structure to initialize.
> + *
> + * If you intend to invoke rcu_head_after_call_rcu() to test whether a
> + * given rcu_head structure has already been passed to call_rcu(), then
> + * you must also invoke this rcu_head_init() function on it just after
> + * allocating that structure.  Calls to this function must not race with
> + * calls to call_rcu(), rcu_head_after_call_rcu(), or callback invocation.
> + */
> +static inline void rcu_head_init(struct rcu_head *rhp)
> +{
> +	rhp->func = (rcu_callback_t)~0L;
> +}
> +
> +/*
> + * rcu_head_after_call_rcu - Has this rcu_head been passed to call_rcu()?
> + * @rhp: The rcu_head structure to test.
> + * @func: The function passed to call_rcu() along with @rhp.
> + *
> + * Returns @true if the @rhp has been passed to call_rcu() with @func,
> + * and @false otherwise.  Emits a warning in any other case, including
> + * the case where @rhp has already been invoked after a grace period.
> + * Calls to this function must not race with callback invocation.  One way
> + * to avoid such races is to enclose the call to rcu_head_after_call_rcu()
> + * in an RCU read-side critical section that includes a read-side fetch
> + * of the pointer to the structure containing @rhp.
> + */
> +static inline bool
> +rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
> +{
> +	if (READ_ONCE(rhp->func) == f)
> +		return true;
> +	WARN_ON_ONCE(READ_ONCE(rhp->func) != (rcu_callback_t)~0L);
> +	return false;
> +}
> +
> +
>  /* Transitional pre-consolidation compatibility definitions. */
>  
>  static inline void synchronize_rcu_bh(void)
> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
> index 5dec94509a7e..4c56c1d98fb3 100644
> --- a/kernel/rcu/rcu.h
> +++ b/kernel/rcu/rcu.h
> @@ -224,6 +224,7 @@ void kfree(const void *);
>   */
>  static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head)
>  {
> +	rcu_callback_t f;
>  	unsigned long offset = (unsigned long)head->func;
>  
>  	rcu_lock_acquire(&rcu_callback_map);
> @@ -234,7 +235,9 @@ static inline bool __rcu_reclaim(const char *rn, struct rcu_head *head)
>  		return true;
>  	} else {
>  		RCU_TRACE(trace_rcu_invoke_callback(rn, head);)
> -		head->func(head);
> +		f = head->func;
> +		WRITE_ONCE(head->func, (rcu_callback_t)0L);
> +		f(head);
>  		rcu_lock_release(&rcu_callback_map);
>  		return false;
>  	}

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