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

Frederic Weisbecker
2022-12-21T11:26:42Z

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
WRITE_ONCE(*IDX, 1);
smp_mb(); // D

// SCAN2
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);
}
}

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