Re: [PATCH 04/10] rcu/nocb: Remove needless full barrier after callback advancing

[Boqun Feng <boqun.feng@gmail.com>]

On Sat, Sep 09, 2023 at 04:31:25AM +0000, Joel Fernandes wrote:
> On Fri, Sep 08, 2023 at 10:35:57PM +0200, Frederic Weisbecker wrote:
> > A full barrier is issued from nocb_gp_wait() upon callbacks advancing
> > to order grace period completion with callbacks execution.
> > 
> > However these two events are already ordered by the
> > smp_mb__after_unlock_lock() barrier within the call to
> > raw_spin_lock_rcu_node() that is necessary for callbacks advancing to
> > happen.
> > 
> > The following litmus test shows the kind of guarantee that this barrier
> > provides:
> > 
> > 	C smp_mb__after_unlock_lock
> > 
> > 	{}
> > 
> > 	// rcu_gp_cleanup()
> > 	P0(spinlock_t *rnp_lock, int *gpnum)
> > 	{
> > 		// Grace period cleanup increase gp sequence number
> > 		spin_lock(rnp_lock);
> > 		WRITE_ONCE(*gpnum, 1);
> > 		spin_unlock(rnp_lock);
> > 	}
> > 
> > 	// nocb_gp_wait()
> > 	P1(spinlock_t *rnp_lock, spinlock_t *nocb_lock, int *gpnum, int *cb_ready)
> > 	{
> > 		int r1;
> > 
> > 		// Call rcu_advance_cbs() from nocb_gp_wait()
> > 		spin_lock(nocb_lock);
> > 		spin_lock(rnp_lock);
> > 		smp_mb__after_unlock_lock();
> > 		r1 = READ_ONCE(*gpnum);
> > 		WRITE_ONCE(*cb_ready, 1);
> > 		spin_unlock(rnp_lock);
> > 		spin_unlock(nocb_lock);
> > 	}
> > 
> > 	// nocb_cb_wait()
> > 	P2(spinlock_t *nocb_lock, int *cb_ready, int *cb_executed)
> > 	{
> > 		int r2;
> > 
> > 		// rcu_do_batch() -> rcu_segcblist_extract_done_cbs()
> > 		spin_lock(nocb_lock);
> > 		r2 = READ_ONCE(*cb_ready);
> > 		spin_unlock(nocb_lock);
> > 
> > 		// Actual callback execution
> > 		WRITE_ONCE(*cb_executed, 1);
> 
> So related to this something in the docs caught my attention under "Callback
> Invocation" [1]
> 
> <quote>
> However, if the callback function communicates to other CPUs, for example,
> doing a wakeup, then it is that function's responsibility to maintain
> ordering. For example, if the callback function wakes up a task that runs on
> some other CPU, proper ordering must in place in both the callback function
> and the task being awakened. To see why this is important, consider the top
> half of the grace-period cleanup diagram. The callback might be running on a
> CPU corresponding to the leftmost leaf rcu_node structure, and awaken a task
> that is to run on a CPU corresponding to the rightmost leaf rcu_node
> structure, and the grace-period kernel thread might not yet have reached the
> rightmost leaf. In this case, the grace period's memory ordering might not
> yet have reached that CPU, so again the callback function and the awakened
> task must supply proper ordering.
> </quote>
> 
> I believe this text is for non-nocb but if we apply that to the nocb case,
> lets see what happens.
> 
> In the litmus, he rcu_advance_cbs() happened on P1, however the callback is
> executing on P2. That sounds very similar to the non-nocb world described in
> the text where a callback tries to wake something up on a different CPU and
> needs to take care of all the ordering.
> 
> So unless I'm missing something (quite possible), P2 must see the update to
> gpnum as well. However, per your limus test, the only thing P2  does is
> acquire the nocb_lock. I don't see how it is guaranteed to see gpnum == 1.

Because P1 writes cb_ready under nocb_lock, and P2 reads cb_ready under
nocb_lock as well and if P2 read P1's write, then we know the serialized
order of locking is P1 first (i.e. the spin_lock(nocb_lock) on P2 reads
from the spin_unlock(nocb_lock) on P1), in other words:

(fact #1)

	unlock(nocb_lock) // on P1
	->rfe
	lock(nocb_lock) // on P2

so if P1 reads P0's write on gpnum

(assumption #1)

	W(gpnum)=1 // on P0
	->rfe
	R(gpnum)=1 // on P1

and we have

(fact #2)

	R(gpnum)=1 // on P1
	->(po; [UL])
	unlock(nocb_lock) // on P1

combine them you get

	W(gpnum)=1 // on P0
	->rfe           // fact #1
	->(po; [UL])    // fact #2
	->rfe           // assumption #1
	lock(nocb_lock) // on P2
	->([LKR]; po)
	M // any access on P2 after spin_lock(nocb_lock);

so
	W(gpnum)=1 // on P0
	->rfe ->po-unlock-lock-po
	M // on P2

and po-unlock-lock-po is A-culum, hence "->rfe ->po-unlock-lock-po" or
"rfe; po-unlock-lock-po" is culum-fence, hence it's a ->prop, which
means the write of gpnum on P0 propagates to P2 before any memory
accesses after spin_lock(nocb_lock)?

Of course, I haven't looked into the bigger picture here (whether the
barrier is for something else, etc.) ;-)

Regards,
Boqun

> I am curious what happens in your litmus if you read gpnum in a register and
> checked for it.
> 
> So maybe the memory barriers you are deleting need to be kept in place? Idk.
> 
> thanks,
> 
>  - Joel
> 
> [1] https://docs.kernel.org/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html#callback-invocation
>