Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

[Peter Zijlstra]

On Fri, Jun 21, 2019 at 12:23:19AM +0200, John Ogness wrote:
> On 2019-06-18, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >> +
> >> +	if (unlikely(newest_id == EOL)) {
> >> +		/* no previous newest means we *are* the list, set oldest */
> >> +
> >> +		/*
> >> +		 * MB UNPAIRED
> >
> > That's a bug, MB must always be paired.
> 
> Well, it "pairs" with the smp_rmb() of the readers, but I didn't think
> that counts as a pair. That's why I wrote unpaired. The litmus test is:
> 
> P0(int *x, int *y)
> {
>         WRITE_ONCE(*x, 1);
>         smp_store_release(y, 1);
> }
> 
> P1(int *x, int *y)
> {
>         int rx;
>         int ry;
> 
>         ry = READ_ONCE(*y);
>         smp_rmb();
>         rx = READ_ONCE(*x);
> }
> 
> exists (1:rx=0 /\ 1:ry=1)
> 
> The readers rely on the store_releases "pairing" with the smp_rmb() so
> that the readers see things in a sane order.

That is certainly a valid pairing, see also the 'SMP BARRIER PAIRING'
section in memory-barriers.txt (I thought we had a table in there, but
apparently that never quite made it in).