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

From: "Paul E. McKenney" <[email-masked]>
Date: Mon, 11 Mar 2019 08:27:52 -0700

On Tue, Jul 31, 2018 at 07:44:29AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 31, 2018 at 03:04:48PM +1000, NeilBrown wrote:
> > 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.
> 
> Now that flaws are to be found, please feel free to report them.  ;-)

Hello, Neil,

Any plans to use these functions?  There are still no upstream uses.
On the other hand, if they proved unuseful, I will remove them.  If I
don't hear otherwise from you, I will pull them in v5.2.

							Thanx, Paul

> > NeilBrown
> > 
> > >
> > > 							Thanx, Paul
> > >
> > > ------------------------------------------------------------------------
> > >
> > > commit e3408141ed7d702995b2fdc94703af88aadd226b
> > > 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 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 <[email-masked]>
> > >     Signed-off-by: Paul E. McKenney <[email-masked]>
> > >     [ 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;
> > >  	}
> 
>