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

[Joel Fernandes <joel@joelfernandes.org>]

On Sat, Sep 09, 2023 at 11:22:48AM -0700, Boqun Feng wrote:
> 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)?

You and Frederic are right. I confirmed this by running herd7 as well.

Also he adds a ->co between P2 and P3, so that's why the
smp_mb__after_lock_unlock() helps to keep the propogation intact. Its pretty
much the R-pattern extended across 4 CPUs.

We should probably document these in the RCU memory ordering docs.

thanks,

 - Joel