>Dosemu does this now on single cpu systems.
Are you sure it's 100% reliable over the time, monotone and with a
picosecond precision and that such way can be extended to SMP?
>gettimeofday always returns a time recently in the past.
It's not a matter of recently in the past, but it must be _monotone_. If
you only lose coherency between tv_sec and tv_usec you can just have a TOD
that goes back instead of forward or equal sometime.
>And having an atomic variable by which keeps a generation number
>for (xtime & other magic constants should be trivial).
>
>Take one snapshot of the generation # at the start of gettimeofday,
>another snapshot at the end of gettimeofday. If the snapshots
>don't match take the slow path, because you've had a race.
read generation (= 1)
rmb()
read rdtsc-ref
read xtime-ref
rmb()
read generation (= 1)
ok now you are happy and you trust the rdtsc-ref and xtime-ref to be
coherent.
But look what was going on in the other CPU0.
CPU0 CPU1
------------------ ---------------------
++generation (= 1)
mb()
write rdtsc-ref
read generation (= 1)
rmb()
read rdtsc-ref
read xtime-ref
rmb()
read generation (= 1)
you are happy but you read a not coherent
value
++generation (= 2)
mb()
write xtime-ref
Maybe there's a trival way to implement the lock-less critical section and
I am missing it...
Andrea
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.rutgers.edu
Please read the FAQ at http://www.tux.org/lkml/