Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

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

On Wed, Dec 21, 2022 at 12:26:29PM +0100, Frederic Weisbecker wrote:
> On Tue, Dec 20, 2022 at 09:41:17PM -0500, Joel Fernandes wrote:
> > 
> > 
> > > On Dec 20, 2022, at 7:50 PM, Frederic Weisbecker <frederic@kernel.org> wrote:
> > > 
> > > On Tue, Dec 20, 2022 at 07:15:00PM -0500, Joel Fernandes wrote:
> > >> On Tue, Dec 20, 2022 at 5:45 PM Frederic Weisbecker <frederic@kernel.org> wrote:
> > >> Agreed about (1).
> > >> 
> > >>> _ In (2), E pairs with the address-dependency between idx and lock_count.
> > >> 
> > >> But that is not the only reason. If that was the only reason for (2),
> > >> then there is an smp_mb() just before the next-scan post-flip before
> > >> the lock counts are read.
> > > 
> > > The post-flip barrier makes sure the new idx is visible on the next READER's
> > > turn, but it doesn't protect against the fact that "READ idx then WRITE lock[idx]"
> > > may appear unordered from the update side POV if there is no barrier between the
> > > scan and the flip.
> > > 
> > > If you remove the smp_mb() from the litmus test I sent, things explode.
> > 
> > Sure I see what you are saying and it’s a valid point as well. However why do you need memory barrier D (labeled such in the kernel code) for that? You already have a memory barrier A before the lock count is read. That will suffice for the ordering pairing with the addr dependency.
> > In other words, if updater sees readers lock counts, then reader would be making those lock count updates on post-flip inactive index, not the one being scanned as you wanted, and you will accomplish that just with the mem barrier A.
> > 
> > So D fixes the above issue you are talking about (lock count update), however that is already fixed by the memory barrier A. But you still need D for the issue I mentioned (unlock counts vs flip).
> > 
> > That’s just my opinion and let’s discuss more because I cannot rule out that I
> > am missing something with this complicated topic ;-)
> 
> I must be missing something. I often do.
> 
> Ok let's put that on litmus:
> 
> ----
> C srcu
> 
> {}
> 
> // updater
> P0(int *IDX, int *LOCK0, int *UNLOCK0, int *LOCK1, int *UNLOCK1)
> {
> 	int lock1;
> 	int unlock1;
> 	int lock0;
> 	int unlock0;
> 
> 	// SCAN1
> 	unlock1 = READ_ONCE(*UNLOCK1);
> 	smp_mb(); // A
> 	lock1 = READ_ONCE(*LOCK1);
> 	
> 	// FLIP
> 	smp_mb(); // E

In real code there is a control dependency between the READ_ONCE() above
and the WRITE_ONCE() before, i.e. only flip the idx when lock1 ==
unlock1, maybe try with the P0 below? Untested due to not having herd on
this computer ;-)

> 	WRITE_ONCE(*IDX, 1);
> 	smp_mb(); // D
> 	
> 	// SCAN2
> 	unlock0 = READ_ONCE(*UNLOCK0);
> 	smp_mb(); // A
> 	lock0 = READ_ONCE(*LOCK0);
> }
> 
	P0(int *IDX, int *LOCK0, int *UNLOCK0, int *LOCK1, int *UNLOCK1)
	{
		int lock1;
		int unlock1;
		int lock0;
		int unlock0;

		// SCAN1
		unlock1 = READ_ONCE(*UNLOCK1);
		smp_mb(); // A
		lock1 = READ_ONCE(*LOCK1);
		
		// FLIP
		if (unlock1 == lock1) {
			smp_mb(); // E
			WRITE_ONCE(*IDX, 1);
			smp_mb(); // D
			
			// SCAN2
			unlock0 = READ_ONCE(*UNLOCK0);
			smp_mb(); // A
			lock0 = READ_ONCE(*LOCK0);
		}
	}

Regards,
Boqun

> // reader
> P1(int *IDX, int *LOCK0, int *UNLOCK0, int *LOCK1, int *UNLOCK1)
> {
> 	int tmp;
> 	int idx;
> 
> 	// 1st reader
> 	idx = READ_ONCE(*IDX);
> 	if (idx == 0) {
> 		tmp = READ_ONCE(*LOCK0);
> 		WRITE_ONCE(*LOCK0, tmp + 1);
> 		smp_mb(); /* B and C */
> 		tmp = READ_ONCE(*UNLOCK0);
> 		WRITE_ONCE(*UNLOCK0, tmp + 1);
> 	} else {
> 		tmp = READ_ONCE(*LOCK1);
> 		WRITE_ONCE(*LOCK1, tmp + 1);
> 		smp_mb(); /* B and C */
> 		tmp = READ_ONCE(*UNLOCK1);
> 		WRITE_ONCE(*UNLOCK1, tmp + 1);
> 	}
> 	
> 	// second reader
> 	idx = READ_ONCE(*IDX);
> 	if (idx == 0) {
> 		tmp = READ_ONCE(*LOCK0);
> 		WRITE_ONCE(*LOCK0, tmp + 1);
> 		smp_mb(); /* B and C */
> 		tmp = READ_ONCE(*UNLOCK0);
> 		WRITE_ONCE(*UNLOCK0, tmp + 1);
> 	} else {
> 		tmp = READ_ONCE(*LOCK1);
> 		WRITE_ONCE(*LOCK1, tmp + 1);
> 		smp_mb(); /* B and C */
> 		tmp = READ_ONCE(*UNLOCK1);
> 		WRITE_ONCE(*UNLOCK1, tmp + 1);
> 	}
> }
> 
> exists (0:lock1!=0)
> ---
> 
> This gives:
> 
> Test srcu Allowed
> States 1
> 0:lock1=0;
> No
> Witnesses
> Positive: 0 Negative: 9
> Condition exists (not (0:lock1=0))
> Observation srcu Never 0 9
> Time srcu 0.57
> Hash=855d17de503814d2421602174f307c59
> 
> Now if I comment out the "smp_mb() /* E */" line this gives:
> 
> Test srcu Allowed
> States 3
> 0:lock1=0;
> 0:lock1=1;
> 0:lock1=2;
> Ok
> Witnesses
> Positive: 4 Negative: 9
> Condition exists (not (0:lock1=0))
> Observation srcu Sometimes 4 9
> 
> Thanks