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

[Mathieu Desnoyers]

----- Original Message -----
> From: "Thomas Gleixner" <tglx@xxxxxxxxxxxxx>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: "LKML" <linux-kernel@xxxxxxxxxxxxxxx>, "John Stultz" <john.stultz@xxxxxxxxxx>, "Peter Zijlstra"
> <peterz@xxxxxxxxxxxxx>, "Steven Rostedt" <rostedt@xxxxxxxxxxx>
> Sent: Saturday, July 12, 2014 4:04:59 PM
> Subject: Re: [patch 54/55] timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC[_RAW]
> 
> 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!

Thanks! Yep, the barriers you propose are what appears
to be missing,

Mathieu

> 
>      tglx
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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