[Devel] Re: [PATCH 2/2] pid: Generalize task_active_pid_ns

[Serge E. Hallyn]

Quoting Sukadev Bhattiprolu (sukadev at linux.vnet.ibm.com):
> From: Sukadev Bhattiprolu <sukadev at linux.vnet.ibm.com>
> Date: Fri, 12 Dec 2008 11:49:27 -0800
> Subject: [PATCH 2/2] pid: Generalize task_active_pid_ns
> 
> Currently task_active_pid_ns is not safe to call after a
> task becomes a zombie and exit_task_namespaces is called,
> as nsproxy becomes NULL.  By reading the pid namespace from
> the pid of the task we can trivially solve this problem at
> the cost of one extra memory read in what should be the
> same cacheline as we read the namespace from.
> 
> When moving things around I have made task_active_pid_ns
> out of line because keeping it in pid_namespace.h would
> require adding includes of pid.h and sched.h that I
> don't think we want.
> 
> This change does make task_active_pid_ns unsafe to call during
> copy_process until we attach a pid on the task_struct which
> seems to be a reasonable trade off.

Yeah that's not really much of a tradeoff...  the text for those
lines gets shorter!  :)

Acked-by: Serge Hallyn <serue at us.ibm.com>

(to all three)

thanks,
-serge

> Signed-off-by: Eric W. Biederman <ebiederm at xmission.com>
> ---
>  include/linux/pid_namespace.h |    6 +-----
>  kernel/fork.c                 |    4 ++--
>  kernel/pid.c                  |    6 ++++++
>  3 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
> index d82fe82..38d1032 100644
> --- a/include/linux/pid_namespace.h
> +++ b/include/linux/pid_namespace.h
> @@ -79,11 +79,7 @@ static inline void zap_pid_ns_processes(struct pid_namespace *ns)
>  }
>  #endif /* CONFIG_PID_NS */
> 
> -static inline struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
> -{
> -	return tsk->nsproxy->pid_ns;
> -}
> -
> +extern struct pid_namespace *task_active_pid_ns(struct task_struct *tsk);
>  void pidhash_init(void);
>  void pidmap_init(void);
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index dba2d3f..9fa1d91 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1100,12 +1100,12 @@ static struct task_struct *copy_process(unsigned long clone_flags,
> 
>  	if (pid != &init_struct_pid) {
>  		retval = -ENOMEM;
> -		pid = alloc_pid(task_active_pid_ns(p));
> +		pid = alloc_pid(p->nsproxy->pid_ns);
>  		if (!pid)
>  			goto bad_fork_cleanup_io;
> 
>  		if (clone_flags & CLONE_NEWPID) {
> -			retval = pid_ns_prepare_proc(task_active_pid_ns(p));
> +			retval = pid_ns_prepare_proc(p->nsproxy->pid_ns);
>  			if (retval < 0)
>  				goto bad_fork_free_pid;
>  		}
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 064e76a..c5513fe 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -474,6 +474,12 @@ pid_t task_session_nr_ns(struct task_struct *tsk, struct pid_namespace *ns)
>  }
>  EXPORT_SYMBOL(task_session_nr_ns);
> 
> +struct pid_namespace *task_active_pid_ns(struct task_struct *tsk)
> +{
> +	return ns_of_pid(task_pid(tsk));
> +}
> +EXPORT_SYMBOL_GPL(task_active_pid_ns);
> +
>  /*
>   * Used by proc to find the first pid that is greater then or equal to nr.
>   *
> -- 
> 1.5.2.5
> 
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