Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

Steven Rostedt
2008-01-16T15:07:00Z

On Wed, 16 Jan 2008, Mathieu Desnoyers wrote:
> Hrm, I will reply to the rest of this email in a separate mail, but
> there is another concern, simpler than memory ordering, that just hit
> me :
>
> If we have CPU A calling clocksource_accumulate while CPU B is calling
> get_monotonic_cycles, but events happens in the following order (because
> of preemption or interrupts). Here, to make things worse, we would be on
> x86 where cycle_t is not an atomic write (64 bits) :
>
>
> CPU A CPU B
>
> clocksource read
> update cycle_mono (1st 32 bits)
> read cycle_mono
> read cycle_last
> clocksource read
> read cycle_mono
> read cycle_last
> update cycle_mono (2nd 32 bits)
> update cycle_last
> update cycle_acc
>
> Therefore, we have :
> - an inconsistant cycle_monotonic value
> - inconsistant cycle_monotonic and cycle_last values.
>
> Or is there something I have missed ?

No, there's probably issues there too, but no need to worry about it,
since I already showed that allowing for clocksource_accumulate to happen
inside the get_monotonic_cycles loop is already flawed.

>
> If you really want an seqlock free algorithm (I _do_ want this for
> tracing!) :) maybe going in the RCU direction could help (I refer to my
> RCU-based 32-to-64 bits lockless timestamp counter extension, which
> could be turned into the clocksource updater).

I know you pointed me the code, but lets assume that I'm still ignorant
;-)

do you actually use the RCU internals? or do you just reimplement an RCU
algorithm?

-- Steve