Subject: Re: [linux-audio-dev] MCS: 64 bit timestamps?
From: David Olofson (audiality_AT_swipnet.se)
Date: su loka 17 1999 - 12:12:23 EDT
On Sun, 17 Oct 1999, Paul Barton-Davis wrote:
> the non-atomic 64 bit stuff is more of a problem. i don't have an easy
> answer yet (or rather, i don't have an answer that doesn't involve
> taking a mutex).
You don't need a mutex for a pointer to a struct... But you *do* need
a bunch of structs in a circular buffer to ensure that the pointer a
thread grabs is valid long enough. It's not sexy, and not even fool
proof (some non-RT task can grab the pointer and go to sleep for a
while...), but it's sync free and simple.
----8<---( mcstime.h )-----------------
struct sync_point_t {
.
.
.
};
extern sync_point_t *mcs_sync_point;
--------------------------------->8----
----8<---( engine.c )------------------
#define SYNCSTRUCTS <sensible value>
static int current_struct = 0;
static sync_point_t struct_tab[SYNCSTRUCTS];
sync_point_t *mcs_sync_point;
void update_sync(void)
{
/* Fill in the new struct... */
.
.
.
struct_tab[current_struct].blah = ...;
/* / / Critical: Atomic pointer write. / / */
mcs_sync_point = &struct_tab[current_struct];
/* / / / / / / / / / / / / / / / / / / / / */
if(++current_struct >= SYNCSTRUCTS)
current_struct = 0;
}
--------------------------------->8----
SYNCSTRUCTS needs to be sync_valid_time / resync_rate for this to be
safe, while soft RT systems and coding style make it impossible to
safely quarantee that the sync_valid_time limit is obeyed...
OTOH, long before that happens, timestamping is already pretty useless
anyway... And, what happens if an structure access collision occurs?
As long as you get a screwed up timestamp, and not a segfault or div
by zero, I'd say it's not a real problem.
//David
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