[PATCH 07/10] posix-timers: Use sighand lock instead of tasklist_lock for task clock sample

Frederic Weisbecker
2013-11-23T15:38:40Z

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 <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
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 7117f6d..cd56e71 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)
--
1.8.3.1