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

Michael Kerrisk mtk.manpages at gmail.com
Wed Jun 25 22:08:14 PDT 2008


On 6/25/08, Nadia.Derbey at bull.net <Nadia.Derbey at bull.net> wrote:
> 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 at bull.net>
>  Signed-off-by: Nadia Derbey <Nadia.Derbey at bull.net>

Nadia,

This is a kernel-userland interface change, please do CC me, so that I
can change the man pages if needed.

Cheers,

Michael



>  ---
>   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
>
>
>  --
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>


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html
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