Messages in this thread | | | Date | Thu, 25 Nov 2010 12:00:13 +0100 | From | Oleg Nesterov <> | Subject | Re: rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create |
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(add Sergey)
On 11/25, Peter Zijlstra wrote: > > On Wed, 2010-11-24 at 20:09 -0500, Dave Jones wrote: > > --- a/kernel/posix-cpu-timers.c > > +++ b/kernel/posix-cpu-timers.c > > @@ -391,6 +391,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > > INIT_LIST_HEAD(&new_timer->it.cpu.entry); > > > > read_lock(&tasklist_lock); > > + rcu_read_lock(); > > if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) { > > if (pid == 0) { > > p = current; > > @@ -414,6 +415,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > > } else { > > ret = -EINVAL; > > } > > + rcu_read_unlock(); > > read_unlock(&tasklist_lock); > > > > return ret; > > Do we still need the tasklist_lock in this case?
No. posix-cpu-timer.c shouldn't use tasklist at all. But it is not completely trivial to remove it.
In particular, this patch is not exactly right, we can't trust thread_group_leader() without tasklist.
Sergey already sent the patch which removes tasklist from posix_cpu_timer_create() and posix_cpu_timer_create(), and iirc Thomas queued it.
> Also, why is that think complaining, surely the tasklist_lock pins any > and all PID objects?
The only problem is: if copy_process() fails, it does free_pid() lockless. This means, without rcu lock it is not safe to scan the rcu-protected lists.
We can change copy_process() (in fact I sent the patch several years ago), but everybody think that find_pid/etc should always take rcu_read_lock() instead. I tend to agree.
Oleg.
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