Fw: [patch] posix timers: RCU optimization for clock_gettime()

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

The attached patch uses RCU to avoid the need to acquire tasklist_lock
in the single-threaded case of clock_gettime().  It still acquires
tasklist_lock when for a (potentially multithreaded) process.  This change
allows realtime applications to frequently monitor CPU consumption of
individual tasks, as requested (and now deployed) by some off-list users.

This has been in Ingo Molnar's -rt patchset since late 2005 with no
problems reported, and tests successfully on 2.6.20-rc6, so I believe
that it is long-since ready for mainline adoption.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---

 kernel/posix-cpu-timers.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Index: linux/kernel/posix-cpu-timers.c
===================================================================
--- linux.orig/kernel/posix-cpu-timers.c
+++ linux/kernel/posix-cpu-timers.c
@@ -304,7 +304,7 @@ int posix_cpu_clock_get(const clockid_t 
 		 * 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)) {
@@ -313,11 +313,13 @@ int posix_cpu_clock_get(const clockid_t 
 								 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)

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