Date:   Wed, 25 Jul 2018 14:53:36 +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 Tue, Jul 24 2018, Paul E. McKenney wrote:

> On Tue, Jul 24, 2018 at 07:52:03AM +1000, NeilBrown wrote:
>> On Mon, Jul 23 2018, Paul E. McKenney wrote:
>> 
>> > On Mon, Jul 23, 2018 at 09:13:43AM +1000, NeilBrown wrote:
>> >> On Sun, Jul 22 2018, Paul E. McKenney wrote:
>> >> >
>> >> > One issue is that the ->func pointer can legitimately be NULL while on
>> >> > RCU's callback lists.  This happens when someone invokes kfree_rcu()
>> >> > with the rcu_head structure at the beginning of the enclosing structure.
>> >> > I could add an offset to avoid this, or perhaps the kmalloc() folks
>> >> > could be persuaded Rao Shoaib's patch moving kfree_rcu() handling to
>> >> > the slab allocators, so that RCU only ever sees function pointers in
>> >> > the ->func field.
>> >> >
>> >> > Either way, this should be hidden behind an API to allow adjustments
>> >> > to be made if needed.  Maybe something like is_after_call_rcu()?
>> >> > This would (for example) allow debug-object checks to be used to catch
>> >> > check-after-free bugs.
>> >> >
>> >> > Would something of that sort work for you?
>> >> 
>> >> Yes, if you could provide an is_after_call_rcu() API, that would
>> >> perfectly suit my use-case.
>> >
>> > After beating my head against the object-debug code a bit, I have to ask
>> > if it would be OK for you if the is_after_call_rcu() API also takes the
>> > function that was passed to RCU.
>> 
>> Sure.  It feels a bit clumsy, but I can see it could be easier to make
>> robust.
>> So yes: I'm fine with pass the same function and rcu_head to both
>> call_rcu() and is_after_call_rcu().  Actually, when I say it like that,
>> it seems less clumsy :-)
>
> How about like this?  (It needs refinements, like lockdep, but should
> get the gist.)
>

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/ ??

Thanks,
NeilBrown


> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 5aa0ebf4799b8bddbbd0124db1c008526e99fc7c
> 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 is_after_call_rcu() and is_after_call_rcu_init()
>     functions to help RCU users detect when another CPU has passed
>     the specified rcu_head structure and function to call_rcu().
>     The is_after_call_rcu_init() should be invoked before making the
>     structure visible to RCU readers, and then the is_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 is_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 is_after_call_rcu_init() has not been invoked on the rcu_head
>     structure or if the rcu_head (AKA callback) has already been invoked,
>     then is_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>
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index e4f821165d0b..82e5a91539b5 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -857,6 +857,45 @@ 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()? */
> +
> +/*
> + * is_after_call_rcu_init - Initialize rcu_head for is_after_call_rcu()
> + * @rhp: The rcu_head structure to initialize.
> + *
> + * If you intend to invoke is_after_call_rcu() to test whether a given
> + * rcu_head structure has already been passed to call_rcu(), then you must
> + * also invoke this is_after_call_rcu_init() function on it just after
> + * allocating that structure.  Calls to this function must not race with
> + * calls to call_rcu(), is_after_call_rcu(), or callback invocation.
> + */
> +static inline void is_after_call_rcu_init(struct rcu_head *rhp)
> +{
> +	rhp->func = (rcu_callback_t)~0L;
> +}
> +
> +/*
> + * is_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 is_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 is_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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