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

[Frederic Weisbecker]

On Tue, Dec 20, 2022 at 11:26:12PM -0500, Joel Fernandes wrote:
> 
> 
> > On Dec 20, 2022, at 10:43 PM, Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> > 
> > On 2022-12-20 19:58, Frederic Weisbecker wrote:
> >>> On Wed, Dec 21, 2022 at 01:49:57AM +0100, Frederic Weisbecker 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@xxxxxxxxxx> 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.
> >> Or rather, look at it the other way, if there is no barrier between the lock
> >> scan and the index flip (E), then the index flip can appear to be written before the
> >> lock is read. Which means you may start activating the index before you finish
> >> reading it (at least it appears that way from the readers pont of view).
> > 
> > Considering that you can have pre-existing readers from arbitrary index appearing anywhere in the grace period (because a reader can fetch the
> > index and be preempted for an arbitrary amount of time before incrementing the lock count), the grace period algorithm needs to deal with the fact that a newcoming reader can appear in a given index either before or after the flip.
> > 
> > I don't see how flipping the index before or after loading the unlock/lock values would break anything (except for unlikely counter overflow situations as previously discussed).
> 
> If you say unlikely, that means it can happen some times which is bad enough ;-). Maybe you mean impossible. I would not settle for anything less than keeping the memory barrier around if it helps unlikely cases, but only D does help for the theoretical wrapping/overflow issue. E is broken and does not even help the theoretical issue IMO. And both D and E do not affect correctness IMO.

And here is why D is needed:

C D

{}

// 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
	WRITE_ONCE(*IDX, 1);
	smp_mb(); // D
	
	// SCAN 2
	unlock0 = READ_ONCE(*UNLOCK0);
	smp_mb(); // A
	lock0 = READ_ONCE(*LOCK0);
}

// 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);
	}
	
	// third 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:unlock0=0 /\ 1:idx=0)

States 6
0:unlock0=0; 1:idx=1;
0:unlock0=1; 1:idx=0;
0:unlock0=1; 1:idx=1;
0:unlock0=2; 1:idx=0;
0:unlock0=2; 1:idx=1;
0:unlock0=3; 1:idx=0;
No
Witnesses
Positive: 0 Negative: 14
Condition exists (0:unlock0=0 /\ 1:idx=0)
Observation D Never 0 14


But then if you comment out "smp_mb() /* D */":

Test D Allowed
States 7
0:unlock0=0; 1:idx=0;
0:unlock0=0; 1:idx=1;
0:unlock0=1; 1:idx=0;
0:unlock0=1; 1:idx=1;
0:unlock0=2; 1:idx=0;
0:unlock0=2; 1:idx=1;
0:unlock0=3; 1:idx=0;
Ok
Witnesses
Positive: 2 Negative: 14
Condition exists (0:unlock0=0 /\ 1:idx=0)
Observation D Sometimes 2 14


Without D I guess things would eventually fix up but that would require an
extra loop in SCAN2.

Thanks.