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

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Message-Id: <20171009161737.ea8c62441cc12dfd909ee0b2@linux-foundation.org>
Date:   Mon, 9 Oct 2017 16:17:37 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Gargi Sharma <gs051095@...il.com>
Cc:     linux-kernel@...r.kernel.org, riel@...riel.com,
        julia.lawall@...6.fr, mingo@...nel.org, pasha.tatashin@...cle.com,
        ktkhai@...tuozzo.com, oleg@...hat.com, ebiederm@...ssion.com,
        hch@...radead.org
Subject: Re: [PATCH v4 1/2] pid: Replace pid bitmap implementation with IDR
 API

On Mon,  9 Oct 2017 17:13:43 -0400 Gargi Sharma <gs051095@...il.com> wrote:

> This patch replaces the current bitmap implemetation for
> Process ID allocation. Functions that are no longer required,
> for example, free_pidmap(), alloc_pidmap(), etc. are removed.
> The rest of the functions are modified to use the IDR API.
> The change was made to make the PID allocation less complex by
> replacing custom code with calls to generic API.
> 

I can't say I really understand the locking here.

>
> ...
>
> @@ -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?