Message-ID: <ZP4kSwWkUdf-rfRW@localhost.localdomain>
Date:   Sun, 10 Sep 2023 22:17:15 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, rcu <rcu@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>
Subject: Re: [PATCH 04/10] rcu/nocb: Remove needless full barrier after
 callback advancing

Le Sun, Sep 10, 2023 at 12:09:23AM -0400, Joel Fernandes a écrit :
> 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.

I have to trust you on that guys, I haven't managed to spend time on
tools/memory-model/Documentation/explanation.txt yet. But glad you sorted
it out.

> 
> thanks,
> 
>  - Joel
> 

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