Re: [RFC PATCH 2/6] IPC/sem: per <pid> semundo file in procfs

["Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

Quoting Nadia.Derbey-6ktuUTfB/bM@public.gmane.org (Nadia.Derbey-6ktuUTfB/bM@public.gmane.org):
> PATCH [02/06]
> 
> This patch adds a new procfs interface to display the per-process semundo
> data. 
> 
> A new per-PID file is added, called "semundo".
> It contains one line per semaphore IPC where there is something to undo for
> this process.
> Then, each line contains the semid followed by each undo value
> corresponding to each semaphores of the semaphores array.
> 
> This interface will be particularly useful to allow a user access
> these data, for example for checkpointing a process
> 
> With this patch, the semundo file can only be accessed in read mode.
> When this file is opened, if an undo_list exists for the target process, it
> is accessed in an rcu read section, and its refcount is incremented, avoiding
> that it be freed.
> The reverse is done during the release operation, and the undo_list is
> freed if the process reading the file was the last process accessing that
> list.
> 
> Signed-off-by: Pierre Peiffer <pierre.peiffer-6ktuUTfB/bM@public.gmane.org>
> Signed-off-by: Nadia Derbey <Nadia.Derbey-6ktuUTfB/bM@public.gmane.org>

Acked-by: Serge Hallyn <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>

-serge

> 
> ---
>  fs/proc/base.c     |    3 +
>  fs/proc/internal.h |    1 
>  ipc/sem.c          |  127 +++++++++++++++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 128 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6.26-rc5-mm3/fs/proc/base.c
> ===================================================================
> --- linux-2.6.26-rc5-mm3.orig/fs/proc/base.c	2008-06-24 09:05:09.000000000 +0200
> +++ linux-2.6.26-rc5-mm3/fs/proc/base.c	2008-06-24 10:03:33.000000000 +0200
> @@ -2525,6 +2525,9 @@ static const struct pid_entry tgid_base_
>  #ifdef CONFIG_TASK_IO_ACCOUNTING
>  	INF("io",	S_IRUGO, tgid_io_accounting),
>  #endif
> +#ifdef CONFIG_SYSVIPC
> +	REG("semundo",  S_IRUGO, semundo),
> +#endif
>  };
> 
>  static int proc_tgid_base_readdir(struct file * filp,
> Index: linux-2.6.26-rc5-mm3/fs/proc/internal.h
> ===================================================================
> --- linux-2.6.26-rc5-mm3.orig/fs/proc/internal.h	2008-06-24 09:05:09.000000000 +0200
> +++ linux-2.6.26-rc5-mm3/fs/proc/internal.h	2008-06-24 10:04:19.000000000 +0200
> @@ -65,6 +65,7 @@ extern const struct file_operations proc
>  extern const struct file_operations proc_net_operations;
>  extern const struct file_operations proc_kmsg_operations;
>  extern const struct inode_operations proc_net_inode_operations;
> +extern const struct file_operations proc_semundo_operations;
> 
>  void free_proc_entry(struct proc_dir_entry *de);
> 
> Index: linux-2.6.26-rc5-mm3/ipc/sem.c
> ===================================================================
> --- linux-2.6.26-rc5-mm3.orig/ipc/sem.c	2008-06-24 09:37:33.000000000 +0200
> +++ linux-2.6.26-rc5-mm3/ipc/sem.c	2008-06-24 10:59:46.000000000 +0200
> @@ -97,6 +97,7 @@ static void freeary(struct ipc_namespace
>  #ifdef CONFIG_PROC_FS
>  static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
>  #endif
> +static void free_semundo_list(struct sem_undo_list *, struct ipc_namespace *);
> 
>  #define SEMMSL_FAST	256 /* 512 bytes on stack */
>  #define SEMOPM_FAST	64  /* ~ 372 bytes on stack */
> @@ -1282,8 +1283,14 @@ void exit_sem(struct task_struct *tsk)
>  	rcu_assign_pointer(tsk->sysvsem.undo_list, NULL);
>  	synchronize_rcu();
> 
> -	if (!atomic_dec_and_test(&ulp->refcnt))
> -		return;
> +	if (atomic_dec_and_test(&ulp->refcnt))
> +		free_semundo_list(ulp, tsk->nsproxy->ipc_ns);
> +}
> +
> +static void free_semundo_list(struct sem_undo_list *ulp,
> +				struct ipc_namespace *ipc_ns)
> +{
> +	BUG_ON(atomic_read(&ulp->refcnt));
> 
>  	for (;;) {
>  		struct sem_array *sma;
> @@ -1303,7 +1310,7 @@ void exit_sem(struct task_struct *tsk)
>  		if (semid == -1)
>  			break;
> 
> -		sma = sem_lock_check(tsk->nsproxy->ipc_ns, un->semid);
> +		sma = sem_lock_check(ipc_ns, un->semid);
> 
>  		/* exit_sem raced with IPC_RMID, nothing to do */
>  		if (IS_ERR(sma))
> @@ -1384,4 +1391,118 @@ static int sysvipc_sem_proc_show(struct 
>  			  sma->sem_otime,
>  			  sma->sem_ctime);
>  }
> +
> +struct undo_list_data {
> +	struct sem_undo_list *undo_list;
> +	struct ipc_namespace *ipc_ns;
> +};
> +
> +/* iterator */
> +static void *semundo_start(struct seq_file *m, loff_t *ppos)
> +{
> +	return NULL;
> +}
> +
> +static void *semundo_next(struct seq_file *m, void *v, loff_t *ppos)
> +{
> +	return NULL;
> +}
> +
> +static void semundo_stop(struct seq_file *m, void *v)
> +{
> +	return;
> +}
> +
> +static int semundo_show(struct seq_file *m, void *v)
> +{
> +	return 0;
> +}
> +
> +static struct seq_operations semundo_op = {
> +	.start	= semundo_start,
> +	.next	= semundo_next,
> +	.stop	= semundo_stop,
> +	.show	= semundo_show
> +};
> +
> +static struct sem_undo_list *get_proc_ulp(struct task_struct *tsk)
> +{
> +	struct sem_undo_list *ulp;
> +
> +	rcu_read_lock();
> +	ulp = rcu_dereference(tsk->sysvsem.undo_list);
> +	if (ulp)
> +		if (!atomic_inc_not_zero(&ulp->refcnt))
> +			ulp = NULL;
> +	rcu_read_unlock();
> +	return ulp;
> +}
> +
> +static void put_proc_ulp(struct sem_undo_list *ulp,
> +					struct ipc_namespace *ns)
> +{
> +	if (ulp && atomic_dec_and_test(&ulp->refcnt))
> +		free_semundo_list(ulp, ns);
> +}
> +
> +/*
> + * semundo_open: open operation for /proc/<PID>/semundo file
> + */
> +static int semundo_open(struct inode *inode, struct file *file)
> +{
> +	struct task_struct *task;
> +	struct sem_undo_list *ulp;
> +	struct undo_list_data *data;
> +	struct ipc_namespace *ns;
> +	int ret = 0;
> +
> +	data = kzalloc(sizeof(*data), GFP_KERNEL);
> +	if (!data)
> +		return -ENOMEM;
> +
> +	task = get_pid_task(PROC_I(inode)->pid, PIDTYPE_PID);
> +	if (!task) {
> +		ret = -EINVAL;
> +		goto out_err;
> +	}
> +
> +	ulp = get_proc_ulp(task);
> +	ns = get_ipc_ns(task->nsproxy->ipc_ns);
> +	put_task_struct(task);
> +
> +	ret = seq_open(file, &semundo_op);
> +	if (!ret) {
> +		struct seq_file *m = file->private_data;
> +		data->undo_list = ulp;
> +		data->ipc_ns = ns;
> +		m->private = data;
> +		return 0;
> +	}
> +
> +	put_proc_ulp(ulp, ns);
> +	put_ipc_ns(ns);
> +out_err:
> +	kfree(data);
> +	return ret;
> +}
> +
> +static int semundo_release(struct inode *inode, struct file *file)
> +{
> +	struct seq_file *m = file->private_data;
> +	struct undo_list_data *data = m->private;
> +	struct sem_undo_list *ulp = data->undo_list;
> +	struct ipc_namespace *ns = data->ipc_ns;
> +
> +	put_proc_ulp(ulp, ns);
> +	put_ipc_ns(ns);
> +	kfree(data);
> +	return seq_release(inode, file);
> +}
> +
> +const struct file_operations proc_semundo_operations = {
> +	.open		= semundo_open,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= semundo_release,
> +};
>  #endif
> 
> --