Re: fork_idle && pid problems ?

Oleg Nesterov wrote:
On 04/17, Ingo Molnar wrote:
* Oleg Nesterov <oleg@xxxxxxxxxx> wrote:

1331 if (likely(p->pid)) {
1351 }
Event leaked_storage: Returned without freeing storage "pid" Also
see events: [alloc_fn][var_assign][pass_arg]
this looks like a false alarm.

p->pid == pid->numbers[0].nr. If "struct pid *pid" was allocated, its
.nr can't be 0.

IOW, !p->pid means that pid == init_struct_pid, it wasn't allocated
but was passed from the caller.
should we perhaps codify this rule via adding something like this to the
else branch:

WARN_ON_ONCE(task_pid(p) != &init_struct_pid);

?

Perhaps yes, I don't know...

But please note that we heavily rely on the fact that nobody except idle
threads can have pid_nr == 0, and more importantly, each "struct pid" must
have the unique .nr withing the same namespace (init_pid_ns in this case).
I'd suggest to just add a small comment.


But wait... What _is_ the task_pid() after fork_idle() ???

It is NULL, but every code getting one can handle such case :)

fork_idle() doesn't really attach the new thread to the init_struct_pid,
so ->pids[PIDTYPE_PID].pid just points the parent's pid, no?

As for x86, the parent is /sbin/init (kernel_init->smp_prepare_cpus),
not so bad, it can't exit.

But what about HOTPLUG_CPU? Suppose we add CPU, use some non-idle
kernel thread (workqueue) to fork the idle thread. CPU goes down,
parent exits and frees the pid. Now, if this CPU goes up again, the
idle thread runs with its ->pid pointing to the freed memory, not
good.

Nope - it will be NULL.

Not serious perhaps, afaics we only need this ->pid to ensure that
swapper can safely fork /sbin/init, but still.

Pavel, Eric, Sukadev? Please say I missed something! ;)

Otherwise, we can change init_idle() to do attach_pid(init_struct_pid),
afaics we can do this lockless. In that case we should also change
INIT_STRUCT_PID() and remove the initialization of .tasks.

Well, these was some request to make tasks always have pid link
point to not NULL (from Matt?) so we'll need this :)

Oleg.



Thanks,
Pavel
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