linux-kernel - Re: [PATCH v2 2/2] pid: Remove pidhash

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Message-ID: <20170927154543.GA3788@redhat.com>
Date:   Wed, 27 Sep 2017 17:45:43 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Gargi Sharma <gs051095@...il.com>
Cc:     linux-kernel@...r.kernel.org, riel@...riel.com,
        julia.lawall@...6.fr, akpm@...ux-foundation.org, mingo@...nel.org,
        pasha.tatashin@...cle.com, ktkhai@...tuozzo.com
Subject: Re: [PATCH v2 2/2] pid: Remove pidhash

On 09/27, Gargi Sharma wrote:
>
> -#define find_next_offset(map, off)					\
> -		find_next_zero_bit((map)->page, BITS_PER_PAGE, off)
> -

this should go into the previous patch, but this is minor...

> @@ -208,12 +200,10 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  
>  	upid = pid->numbers + ns->level;
>  	spin_lock_irq(&pidmap_lock);
> -	if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
> +	if (!(ns->pid_allocated & PIDNS_ADDING))
>  		goto out_unlock;
>  	for ( ; upid >= pid->numbers; --upid) {
> -		hlist_add_head_rcu(&upid->pid_chain,
> -				&pid_hash[pid_hashfn(upid->nr, upid->ns)]);
> -		upid->ns->nr_hashed++;
> +		upid->ns->pid_allocated++;

No, this is wrong.

It is too late to check PIDNS_HASH_ADDING/PIDNS_ADDING and increment pid_allocated,
once we call idr_alloc_cyclic() this pid is already "hashed" in that it can be found
by find_pid_ns() with this patch applied.

And of course, it is too late to do atomic_set(&pid->count, 1) and initialize
pid->tasks[type] lists by the same reason.

Oleg.