* Question about replacing while_each_thread().
@ 2017-02-01 10:47 Tetsuo Handa
2017-02-01 17:19 ` Oleg Nesterov
0 siblings, 1 reply; 2+ messages in thread
From: Tetsuo Handa @ 2017-02-01 10:47 UTC (permalink / raw)
To: oleg; +Cc: dserrg, snanda, rientjes, linux-kernel
Hello.
I have a question about commit 0c740d0afc3bff0a ("introduce
for_each_thread() to replace the buggy while_each_thread()").
IOPRIO_WHO_USER case in sys_ioprio_set()/sys_ioprio_get() in block/ioprio.c
are using
rcu_read_lock();
do_each_thread(g, p) {
(...snipped...)
} while_each_thread(g, p);
rcu_read_unlock();
sequence which is unsafe according to that commit, but
I'm not sure what the correct fix is.
That commit says
The new for_each_thread(g, t) helper is always safe under
rcu_read_lock() as long as this task_struct can't go away.
but what is the requirement for "can't go away" ?
Is rcu_read_lock() sufficient (i.e.
rcu_read_lock();
for_each_process_thread(g, p) {
(...snipped...)
}
rcu_read_unlock();
is OK) for "can't go away" ?
Is tasklist_lock held for read or write required (i.e.
read_lock(&tasklist_lock);
for_each_process_thread(g, p) {
(...snipped...)
}
read_unlock(&tasklist_lock);
is needed) for "can't go away" ?
I hope rcu_read_lock() is sufficient according to usage in
show_state_filter() and check_hung_uninterruptible_tasks().
Likewise, IOPRIO_WHO_PGRP case are using
rcu_read_lock();
do {
if ((pgrp) != NULL)
hlist_for_each_entry_rcu((p), &(pgrp)->tasks[PIDTYPE_PGID], pids[PIDTYPE_PGID].node) {
{
struct task_struct *tg___ = p;
do {
(...snipped...)
} while_each_thread(tg___, p);
p = tg___;
}
if (PIDTYPE_PGID == PIDTYPE_PID)
break;
}
} while (0);
rcu_read_unlock();
sequence which I guess it is unsafe as well.
In this case updating do_each_pid_thread() to use for_each_thread() and
updating while_each_pid_thread() not to use while_each_thread() is
the correct fix?
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Question about replacing while_each_thread().
2017-02-01 10:47 Question about replacing while_each_thread() Tetsuo Handa
@ 2017-02-01 17:19 ` Oleg Nesterov
0 siblings, 0 replies; 2+ messages in thread
From: Oleg Nesterov @ 2017-02-01 17:19 UTC (permalink / raw)
To: Tetsuo Handa; +Cc: dserrg, snanda, rientjes, linux-kernel
Hi Tetsuo,
On 02/01, Tetsuo Handa wrote:
>
> Is rcu_read_lock() sufficient (i.e.
>
> rcu_read_lock();
> for_each_process_thread(g, p) {
> (...snipped...)
> }
> rcu_read_unlock();
>
> is OK) for "can't go away" ?
Yes, this should work just fine,
> Likewise, IOPRIO_WHO_PGRP case are using
>
> rcu_read_lock();
> do {
> if ((pgrp) != NULL)
> hlist_for_each_entry_rcu((p), &(pgrp)->tasks[PIDTYPE_PGID], pids[PIDTYPE_PGID].node) {
> {
> struct task_struct *tg___ = p;
> do {
> (...snipped...)
> } while_each_thread(tg___, p);
> p = tg___;
> }
> if (PIDTYPE_PGID == PIDTYPE_PID)
> break;
> }
> } while (0);
> rcu_read_unlock();
>
> sequence which I guess it is unsafe as well.
Hmm, indeed, I forgot there is another while_each_thread() hidden in
do_each_pid_thread()
> In this case updating do_each_pid_thread() to use for_each_thread() and
> updating while_each_pid_thread() not to use while_each_thread() is
> the correct fix?
Yes, I think so, just
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -191,10 +191,10 @@ pid_t pid_vnr(struct pid *pid);
#define do_each_pid_thread(pid, type, task) \
do_each_pid_task(pid, type, task) { \
struct task_struct *tg___ = task; \
- do {
+ for_each_thread(tg__, task) {
#define while_each_pid_thread(pid, type, task) \
- } while_each_thread(tg___, task); \
+ } \
task = tg___; \
} while_each_pid_task(pid, type, task)
#endif /* _LINUX_PID_H */
but perhaps we can also cleanup it... the usage of 'tg___' doesn't look nice
either way, so perhaps
#define do_each_pid_thread(pid, type, task) do { \
struct task_struct *tg___; \
do_each_pid_task(pid, type, tg___) { \
for_each_thread(tg__, task) {
#define while_each_pid_thread(pid, type, task) \
} \
} while_each_pid_task(pid, type, task); \
} while (0)
will look a bit better? up to you.
Oleg.
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