linux-kernel - Re: [PATCH 1/1] kernel/pid.c: Masking the flag out to get the actual value.

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Date:	Tue, 11 Jun 2013 15:45:18 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Raphael S. Carvalho" <raphael.scarv@...il.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] kernel/pid.c: Masking the flag out to get the
 actual value.

On Mon, 10 Jun 2013 18:56:38 -0300 "Raphael S. Carvalho" <raphael.scarv@...il.com> wrote:

> This patch shouldn't be applied if those branches must only be taken when
> the pid_allocation(PIDNS_HASH_ADDING) flag was turned off.

Well yes - hopefully this is the case.  Otherwise we're missing some
rather important-looking wakeups.


> Otherwise, we must turn the PIDNS_HASH_ADDING flag (1U << 31) off
> before getting into the switch-cases.
> 
> ...
>
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -263,7 +263,10 @@ void free_pid(struct pid *pid)
>  		struct upid *upid = pid->numbers + i;
>  		struct pid_namespace *ns = upid->ns;
>  		hlist_del_rcu(&upid->pid_chain);
> -		switch(--ns->nr_hashed) {
> +
> +		/* We must turn the PIDNS_HASH_ADDING flag off to
> +		   get the actual value of nr_hashed */
> +		switch ((--ns->nr_hashed) & ~(PIDNS_HASH_ADDING)) {
>  		case 1:
>  			/* When all that is left in the pid namespace
>  			 * is the reaper wake up the reaper.  The reaper

Eric, can you please take a look?  Presumably and hopefully
PIDNS_HASH_ADDING cannot be set here, but what guarantees this?

Hopefully we can fix this one by adding the missing comment.
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