Hi!
We need an atomic portable library. This only works for gcc and the
gcc in OSX is tool old. I am keeping it in my tree, but mainly for my
pedantic needs.
Cheers,
-Brian
On Feb 18, 2008, at 3:49 PM, dormando wrote:
Has anyone else tested this patch?
Does it work with ancient gcc? Anyone from the solaris camp want to
comment?
I might save this for the next cycle, because:
1) Failure causes the old value to be read until the CPU cache is
synced
completely. Not a huge deal. We'll get way more benefits changing some
of the more inane mutexes into atomic CAS.
2) Potential compatibility breakage. Once I have buildbot up and
running
we can try pushing code like this around and see what it pisses off :)
3) Not entirely sure the while loop is right? Must research...
-Dormando
Brian Aker wrote:
Hi!
Here is a patch to fix the problem with concurrency in the
set_current_time() call.
diff -r 1d3a59c1dbd3 memcached.c
--- a/memcached.c Mon Feb 04 15:07:29 2008 -0800
+++ b/memcached.c Mon Feb 04 17:33:21 2008 -0800
@@ -2560,7 +2560,9 @@ static struct event clockevent;
/* time-sensitive callers can call it by hand with this, outside the
normal ever-1-second timer */
static void set_current_time(void) {
- current_time = (rel_time_t) (time(0) - stats.started);
+ volatile rel_time_t new_time;
+ new_time = (rel_time_t) (time(0) - stats.started);
+ while(!( __sync_bool_compare_and_swap(¤t_time,
current_time,
new_time)));
}
The problem will be experienced when you are on a dual processor
machine
(aka not a dual core (for most vendors) just a dual processor). The
volatile declaration only helps with caching, it does not solve the
problem where one processor will see one thing, while another will
see a
different value during the setting of the variable.
One note about the above, I did not use a mutex since this variable
would then have to be locked in all sections of the code (which
would be
slow...). This is a gcc specific fix BTW. If others are interested we
can put together an include file for different platforms.
Cheers,
-Brian
--
_______________________________________________________
Brian "Krow" Aker, brian at tangent.org
Seattle, Washington
http://krow.net/ <-- Me
http://tangent.org/ <-- Software
http://exploitseattle.com/ <-- Fun
_______________________________________________________
You can't grep a dead tree.
--
_______________________________________________________
Brian "Krow" Aker, brian at tangent.org
Seattle, Washington
http://krow.net/ <-- Me
http://tangent.org/ <-- Software
http://exploitseattle.com/ <-- Fun
_______________________________________________________
You can't grep a dead tree.
