----- Original Message -----
> From: "Thomas Gleixner" <[email protected]>
> To: "LKML" <[email protected]>
> Cc: "John Stultz" <[email protected]>, "Peter Zijlstra"
> <[email protected]>, "Steven Rostedt"
> <[email protected]>, "Mathieu Desnoyers" <[email protected]>
> Sent: Friday, July 11, 2014 9:45:19 AM
> Subject: [patch 54/55] timekeeping: Provide fast and NMI safe access to
> CLOCK_MONOTONIC[_RAW]
>
> Tracers want a correlated time between the kernel instrumentation and
> user space. We really do not want to export sched_clock() to user
> space, so we need to provide something sensible for this.
>
> Using separate data structures with an non blocking sequence count
> based update mechanism allows us to do that. The data structure
> required for the readout has a sequence counter and two copies of the
> timekeeping data.
>
> On the update side:
>
> tkf->seq++;
> smp_wmb();
> update(tkf->base[0], tk;
> tkf->seq++;
> smp_wmb();
> update(tkf->base[1], tk;
>
> On the reader side:
>
> do {
> seq = tkf->seq;
> smp_rmb();
> idx = seq & 0x01;
> now = now(tkf->base[idx]);
> smp_rmb();
> } while (seq != tkf->seq)
>
> So if NMI hits the update of base[0] it will use base[1] which is
> still consistent. In case of CLOCK_MONOTONIC this can result in
> slightly wrong timestamps (a few nanoseconds) accross an update. Not a
> big issue for the intended use case.
Hi Thomas,
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 ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
