Subject: Re: [PATCH 2/5] rhashtable: don't hold lock on first table throughout insertion.

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 25 Jul 2018 08:22:50 -0700

On Wed, Jul 25, 2018 at 02:53:36PM +1000, NeilBrown wrote:
> 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/ ??

How about rcu_head_init() and rcu_head_after_call_rcu()?

							Thanx, Paul

> Thanks,
> NeilBrown
> 
> 
> > 							Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit 5aa0ebf4799b8bddbbd0124db1c008526e99fc7c
> > Author: Paul E. McKenney <[email-masked]>
> > 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 <[email-masked]>
> >     Signed-off-by: Paul E. McKenney <[email-masked]>
> >
> > 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;
> >  	}