From: "Paul E. McKenney" 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. Signed-off-by: Paul E. McKenney Signed-off-by: Ingo Molnar Cc: Thomas Gleixner Cc: john stultz Cc: Roman Zippel Cc: Oleg Nesterov Signed-off-by: Andrew Morton --- kernel/posix-cpu-timers.c | 6 ++++-- 1 files changed, 4 insertions(+), 2 deletions(-) diff -puN kernel/posix-cpu-timers.c~posix-timers-rcu-optimization-for-clock_gettime kernel/posix-cpu-timers.c --- a/kernel/posix-cpu-timers.c~posix-timers-rcu-optimization-for-clock_gettime +++ a/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) _