Re: How to read xtime

Andrea Arcangeli (
Fri, 26 Feb 1999 19:08:19 +0100 (CET)

On Fri, 26 Feb 1999, Colin Plumb wrote:

>> I think the issue is timer_reg. What is timer_reg? To avoid a xtime_lock
>> we must make sure that the other CPU is not in the middle of the xtime
>> updating (so that only half of the xtime struct is been updated).
>Umm if you can write to it atomically, that solves everything.
>And I think I have the (x86) solution.... cmpxchg8b.

Ignoring the 486 issue...:

Does it really worth the pain? Note that in gettimeofday we need the
spinlock _anyway_ because we could race between the timer bh handler and
gettimeofday (called from an irq via get_fast_time()) (see lostticks in
sched.c and arch/i386/kernel/time.c).

And I see netif_rx() as the piece of code that play with xtime of sytem
more heavily and since for it we still need the read_write_spinlock I
don't think it worth to optimize the far less frequent get_xtime() case
(get_xtime() in my tree is a normal `2.2.2 xtime read', but protected by
the read_lock/unlock(&xtime_lock) to get a 100% sure coherent xtime

I think it would increase still more the complexity of the xtime handling
without a real benefit (and btw 2.2.2 would benefit _zero_ from such asm
cleverness, since it just bugs and doesn't use the read-spinlock around
reading xtime as I do here).

Andrea Arcangeli

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