Concurrency Bug in memcached

Brian Aker brian at
Tue Feb 5 01:42:54 UTC 2008


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(&current_time, current_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.


Brian "Krow" Aker, brian at
Seattle, Washington                     <-- Me                <-- Software    <-- Fun
You can't grep a dead tree.

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