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


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
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