From 50875788a1d4a3f662a27ed13cd05282d835939a Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Fri, 11 Oct 2013 17:41:11 +0200 Subject: [PATCH] posix-timers: Use sighand lock instead of tasklist_lock for task clock sample There is no need for the tasklist_lock just to take a process wide clock sample. All we need is to get a coherent sample that doesn't race with exit() and exec(): * exit() may be concurrently reaping a task and flushing its time * sighand is unstable under exit() and exec(), and the latter also result in group leader that can change To protect against these, locking the target's sighand is enough. Signed-off-by: Frederic Weisbecker Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Peter Zijlstra Cc: Oleg Nesterov Cc: Kosaki Motohiro Cc: Andrew Morton --- kernel/posix-cpu-timers.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c index 03c5d6c3e614..71a07699a36b 100644 --- a/kernel/posix-cpu-timers.c +++ b/kernel/posix-cpu-timers.c @@ -271,12 +271,22 @@ static int posix_cpu_clock_get_task(struct task_struct *tsk, if (same_thread_group(tsk, current)) err = cpu_clock_sample(which_clock, tsk, &rtn); } else { - read_lock(&tasklist_lock); + unsigned long flags; + struct sighand_struct *sighand; - if (tsk->sighand && (tsk == current || thread_group_leader(tsk))) + /* + * while_each_thread() is not yet entirely RCU safe, + * keep locking the group while sampling process + * clock for now. + */ + sighand = lock_task_sighand(tsk, &flags); + if (!sighand) + return err; + + if (tsk == current || thread_group_leader(tsk)) err = cpu_clock_sample_group(which_clock, tsk, &rtn); - read_unlock(&tasklist_lock); + unlock_task_sighand(tsk, &flags); } if (!err)