On Fri, Jan 18, 2002 at 12:36:00PM -0800, David S. Miller wrote:
> Andi, the x86 platforms that have SMP have TSC and in that
> case do_get_fast_time == do_gettimeofday and there is no read locking
> :-))))))))))))
Unless I'm misreading the code completely there is readlocking:
void do_gettimeofday(struct timeval *tv)
{
unsigned long flags;
unsigned long usec, sec;
read_lock_irqsave(&xtime_lock, flags);
usec = do_gettimeoffset();
...
if (cpu_has_tsc) {
unsigned long tsc_quotient = calibrate_tsc();
if (tsc_quotient) {
...
do_get_fast_time = do_gettimeofday;
void (*do_get_fast_time)(struct timeval *) = do_normal_gettime;
/*
* Generic way to access 'xtime' (the current time of day).
* This can be changed if the platform provides a more accurate (and fast!)
* version.
*/
void get_fast_time(struct timeval * t)
{
do_get_fast_time(t);
}
There would be no locking if get_fast_time used do_gettimeoffset, but it doesn't.
The only lockless i386 gettimeofday i'm aware of is the one in the x86-64 vsyscall
implementation from Andrea.
Anyways. I guess it would hurt to change get_fast_time to gettimeoffset because
it could cause non-monotonies for software that is comparing the socket timestamp
with gettimeofday. A better plan might be to port Andrea's version over to
the generic TSC do_gettimeofday and avoid the lock.
-Andi
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