linux-kernel - Re: [PATCH v4 1/2] pid: Replace pid bitmap implementation with IDR API

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Date:   Tue, 10 Oct 2017 13:35:23 +0100
From:   Gargi Sharma <gs051095@...il.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Rik van Riel <riel@...riel.com>,
        Julia Lawall <julia.lawall@...6.fr>, mingo@...nel.org,
        pasha.tatashin@...cle.com, ktkhai@...tuozzo.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v4 1/2] pid: Replace pid bitmap implementation with IDR API

On Tue, Oct 10, 2017 at 12:50 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 10/09, Andrew Morton wrote:
>>
>> > @@ -240,17 +230,11 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
>> >      *
>> >      */
>> >     read_lock(&tasklist_lock);
>> > -   nr = next_pidmap(pid_ns, 1);
>> > -   while (nr > 0) {
>> > -           rcu_read_lock();
>> > -
>> > -           task = pid_task(find_vpid(nr), PIDTYPE_PID);
>> > +   nr = 2;
>> > +   idr_for_each_entry_continue(&pid_ns->idr, pid, nr) {
>> > +           task = pid_task(pid, PIDTYPE_PID);
>> >             if (task && !__fatal_signal_pending(task))
>> >                     send_sig_info(SIGKILL, SEND_SIG_FORCED, task);
>> > -
>> > -           rcu_read_unlock();
>> > -
>> > -           nr = next_pidmap(pid_ns, nr);
>> >     }
>> >     read_unlock(&tasklist_lock);
>>
>> Especially here.  I don't think pidmap_lock is held.  Is that IDR
>> iteration safe?
>
> Yes, this doesn't look right, we need rcu_read_lock() or pidmap_lock.
>
> And, we also need rcu_read_lock() for another reason, to protect "struct pid".

Ah, I missed this. From what I understood idr_for_each_entry_continue
should be safe because calls idr_get_next which in turn calls
radix_tree_iter_find to find the next populated entry in the idr. If
the pid that you are looking up the task for is deleted, task will get
a NULL from pid_task and no signal to kill will be sent.
>
> Gargi, I suggested to use idr_for_each_entry_continue(), but now I am wondering
> if we should use idr_for_each() instead. IIUC this would be a bit faster? Not
> that I think this is really important...

I can run benchmarks with idr_for_each to see how much speed up is
achieved and then we can go with whatever we think is better. How does
that sounds?

Thanks!
Gargi
>
> Oleg.
>