[PATCH] posix timers: RCU optimization for clock_gettime()

Use 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.

[paulmck@linux.vnet.ibm.com: fix exit()/posix_cpu_clock_get() race spotted by Oleg]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 5 deletions

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 7c3e1e6dfb5b..657f77697415 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -304,7 +304,7 @@ int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
                  * 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)) {
@@ -312,12 +312,17 @@ int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
                                         error = cpu_clock_sample(which_clock,
                                                                  p, &rtn);
                                 }
-                        } else if (p->tgid == pid && p->signal) {
-                                error = cpu_clock_sample_group(which_clock,
-                                                               p, &rtn);
+                        } else {
+                                read_lock(&tasklist_lock);
+                                if (p->tgid == pid && p->signal) {
+                                        error =
+                                            cpu_clock_sample_group(which_clock,
+                                                                   p, &rtn);
+                                }
+                                read_unlock(&tasklist_lock);
                         }
                 }
-                read_unlock(&tasklist_lock);
+                rcu_read_unlock();
         }
 
         if (error)