Re: [patch 54/55] timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC[_RAW]

[Thomas Gleixner]

On Sat, 12 Jul 2014, Thomas Gleixner wrote:
> On Sat, 12 Jul 2014, Mathieu Desnoyers wrote:
> > I'm perhaps missing something here, but what happens with the
> > following scenario ?
> > 
> > Initial conditions:
> > 
> > tkf->seq = 0
> > tkf->base[0] and tkf->base[1] are initialized.
> > 
> > CPU 0                                      CPU 1
> > ------------                               ----------------
> > update:
> >   tkf->seq++
> >   smb_wmb()
> >   tkf->seq++ (reordered before update)
> >                                            reader:
> >                                            seq = tkf->seq (reads 2)
> >                                            smp_rmb()
> >                                            idx = seq & 0x01
> >                                            now = now(tkf->base[idx]   (reads base[0])
> >   update(tkf->base[0], tk) (racy concurrent update)
> >                                            smp_rmb()
> >                                            while (seq != tkf->seq) (they are equal)
> > 
> > So AFAIU, we end up returning a corrupted value. Adding a
> > smp_wmb() between update of base[0] and increment of seq,
> > as well as between update of base[1] and the _following_
> > increment of seq (next update call) would fix this.
> > 
> > Thoughts ?

Second thoughts :)

> Well, the actual implementation does:
> 
> +       /* Force readers off to base[1] */
> +       raw_write_seqcount_begin(&tkf->seq);

i.e:
	seq++;
	smp_wmb();

> +
> +       /* Update base[0] */
> +       base->clock = clk;
> +       base->cycle_last = clk->cycle_last;
> +       base->base = tbase;
> +       base->shift = shift;
> +       base->mult = mult;
> +
> +       /* Force readers back to base[0] */
> +       raw_write_seqcount_end(&tkf->seq);

i.e:
	smp_wmb();
	seq++;

So while this orders against the update of base0, it does not prevent
reordering against the update of base1. So you're right, we need a

  	smp_wmb();

before we start updating base1.

> +       /* Update base[1] */
> +       base++;
> +       base->clock = clk;
> +       base->cycle_last = clk->cycle_last;
> +       base->base = tbase;
> +       base->shift = shift;
> +       base->mult = mult;

So as a consequence we need another one here:

      	smp_wmb();

to protect against the unlikely, but possible seq++ at the begin of
the update. Debatable whether this can happen without another wmb()
between the two calls, but yes for sanity reasons we should add it
until we can prove that the actual call chains prevent this.

Nice catch!

     tglx
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