[RFC,PATCH] RCUify single-thread case of clock_gettime()

["Paul E. McKenney" <paulmck@us.ibm.com>]

Hello!

The attached patch uses RCU to avoid the need to acquire tasklist_lock
in the single-thread case of clock_gettime().  Still acquires tasklist_lock
when asking for the time of a (potentially multithreaded) process.

Experimental, has been touch-tested on x86 and POWER.  Requires RCU on
task_struct.  Further more focused testing in progress.

Thoughts?  (Why?  Some off-list users want to be able to monitor CPU
consumption of specific threads.  They need to do so quite frequently,
so acquiring tasklist_lock is inappropriate.)

							Thanx, Paul

diff -urpNa -X dontdiff linux-2.6.14-rc5-rt2/kernel/posix-cpu-timers.c linux-2.6.14-rc5-rt2-RCUgettime/kernel/posix-cpu-timers.c
--- linux-2.6.14-rc5-rt2/kernel/posix-cpu-timers.c	2005-10-22 14:45:56.000000000 -0700
+++ linux-2.6.14-rc5-rt2-RCUgettime/kernel/posix-cpu-timers.c	2005-10-22 15:44:27.000000000 -0700
@@ -302,7 +302,7 @@ int posix_cpu_clock_get(clockid_t which_
 		 * should be able to see it.
 		 */
 		struct task_struct *p;
-		read_lock(&tasklist_lock);
+		rcu_read_lock();
 		p = find_task_by_pid(pid);
 		if (p) {
 			if (CPUCLOCK_PERTHREAD(which_clock)) {
@@ -311,11 +311,13 @@ int posix_cpu_clock_get(clockid_t which_
 								 p, &rtn);
 				}
 			} else if (p->tgid == pid && p->signal) {
+				read_lock(&tasklist_lock);
 				error = cpu_clock_sample_group(which_clock,
 							       p, &rtn);
+				read_unlock(&tasklist_lock);
 			}
 		}
-		read_unlock(&tasklist_lock);
+		rcu_read_unlock();
 	}
 
 	if (error)