[Devel] [RFC PATCH 3/6] IPC/sem: start/stop operations for /proc/pid/semundo

[Nadia.Derbey at bull.net]

PATCH [03/06]

This patch introduces the .start and .stop seq operations for
/proc/pid/semundo.

Signed-off-by: Pierre Peiffer <pierre.peiffer at bull.net>
Signed-off-by: Nadia Derbey <Nadia.Derbey at bull.net>

---
 ipc/sem.c |   43 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 2 deletions(-)

Index: linux-2.6.26-rc5-mm3/ipc/sem.c
===================================================================
--- linux-2.6.26-rc5-mm3.orig/ipc/sem.c	2008-06-24 10:59:46.000000000 +0200
+++ linux-2.6.26-rc5-mm3/ipc/sem.c	2008-06-24 12:32:36.000000000 +0200
@@ -1400,7 +1400,42 @@ struct undo_list_data {
 /* iterator */
 static void *semundo_start(struct seq_file *m, loff_t *ppos)
 {
-	return NULL;
+	struct undo_list_data *data = m->private;
+	struct sem_undo_list *ulp = data->undo_list;
+	struct sem_undo	*undo;
+	loff_t pos = *ppos;
+
+	if (!ulp)
+		return NULL;
+
+	if (pos < 0)
+		return NULL;
+
+	/* If ulp is not NULL, it means that we've successfully grabbed
+	 * a refcnt in semundo_open. That prevents the undo_list from being
+	 * freed.
+	 *
+	 * Note:
+	 * 1) the sem_undo structure is RCU-protected. So take the rcu read
+	 *    lock and only release it during the .stop operation.
+	 * 2) we accept to release the undo_list lock (i.e. we allow the list
+	 *    to change) between each iteration, instead of taking that lock
+	 *    during the .start and releasing it during the .stop operation.
+	 *    This is to reduce the performance impact on the access to the
+	 *    undo_list.
+	 */
+	rcu_read_lock();
+	spin_lock(&ulp->lock);
+	list_for_each_entry_rcu(undo, &ulp->list_proc, list_proc) {
+		if ((undo->semid != -1) && !(pos--))
+			break;
+	}
+	spin_unlock(&ulp->lock);
+
+	if (&undo->list_proc == &ulp->list_proc)
+		return NULL;
+
+	return undo;
 }
 
 static void *semundo_next(struct seq_file *m, void *v, loff_t *ppos)
@@ -1410,7 +1445,11 @@ static void *semundo_next(struct seq_fil
 
 static void semundo_stop(struct seq_file *m, void *v)
 {
-	return;
+	struct undo_list_data *data = m->private;
+	struct sem_undo_list *ulp = data->undo_list;
+
+	if (ulp)
+		rcu_read_unlock();
 }
 
 static int semundo_show(struct seq_file *m, void *v)

--
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