ipc: re-enable msgmni automatic recomputing msgmni if set to negative

The enhancement as asked for by Yasunori: if msgmni is set to a negative
value, register it back into the ipcns notifier chain.

A new interface has been added to the notification mechanism:
notifier_chain_cond_register() registers a notifier block only if not already
registered.  With that new interface we avoid taking care of the states
changes in procfs.

Signed-off-by: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Mingming Cao <cmm@us.ibm.com>
Cc: Pierre Peiffer <pierre.peiffer@bull.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

5 files changed, 87 insertions, 10 deletions

diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
index c3b1da9e5feb..ea6c18a8b0d4 100644
--- a/include/linux/ipc_namespace.h
+++ b/include/linux/ipc_namespace.h
@@ -52,6 +52,7 @@ extern atomic_t nr_ipc_ns;
 #define INIT_IPC_NS(ns)         .ns             = &init_ipc_ns,
 
 extern int register_ipcns_notifier(struct ipc_namespace *);
+extern int cond_register_ipcns_notifier(struct ipc_namespace *);
 extern int unregister_ipcns_notifier(struct ipc_namespace *);
 extern int ipcns_notify(unsigned long);
 
diff --git a/include/linux/notifier.h b/include/linux/notifier.h
index 20dfed590183..0ff6224d172a 100644
--- a/include/linux/notifier.h
+++ b/include/linux/notifier.h
@@ -121,6 +121,10 @@ extern int raw_notifier_chain_register(struct raw_notifier_head *nh,
 extern int srcu_notifier_chain_register(struct srcu_notifier_head *nh,
                 struct notifier_block *nb);
 
+extern int blocking_notifier_chain_cond_register(
+                struct blocking_notifier_head *nh,
+                struct notifier_block *nb);
+
 extern int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh,
                 struct notifier_block *nb);
 extern int blocking_notifier_chain_unregister(struct blocking_notifier_head *nh,
diff --git a/ipc/ipc_sysctl.c b/ipc/ipc_sysctl.c
index d12ff5cd2a0b..d3497465cc0a 100644
--- a/ipc/ipc_sysctl.c
+++ b/ipc/ipc_sysctl.c
@@ -15,6 +15,8 @@
 #include <linux/sysctl.h>
 #include <linux/uaccess.h>
 #include <linux/ipc_namespace.h>
+#include <linux/msg.h>
+#include "util.h"
 
 static void *get_ipc(ctl_table *table)
 {
@@ -24,6 +26,27 @@ static void *get_ipc(ctl_table *table)
         return which;
 }
 
+/*
+ * Routine that is called when a tunable has successfully been changed by
+ * hand and it has a callback routine registered on the ipc namespace notifier
+ * chain: we don't want such tunables to be recomputed anymore upon memory
+ * add/remove or ipc namespace creation/removal.
+ * They can come back to a recomputable state by being set to a <0 value.
+ */
+static void tunable_set_callback(int val)
+{
+        if (val >= 0)
+                unregister_ipcns_notifier(current->nsproxy->ipc_ns);
+        else {
+                /*
+                 * Re-enable automatic recomputing only if not already
+                 * enabled.
+                 */
+                recompute_msgmni(current->nsproxy->ipc_ns);
+                cond_register_ipcns_notifier(current->nsproxy->ipc_ns);
+        }
+}
+
 #ifdef CONFIG_PROC_FS
 static int proc_ipc_dointvec(ctl_table *table, int write, struct file *filp,
         void __user *buffer, size_t *lenp, loff_t *ppos)
@@ -38,17 +61,17 @@ static int proc_ipc_dointvec(ctl_table *table, int write, struct file *filp,
 static int proc_ipc_callback_dointvec(ctl_table *table, int write,
         struct file *filp, void __user *buffer, size_t *lenp, loff_t *ppos)
 {
+        struct ctl_table ipc_table;
         size_t lenp_bef = *lenp;
         int rc;
 
-        rc = proc_ipc_dointvec(table, write, filp, buffer, lenp, ppos);
+        memcpy(&ipc_table, table, sizeof(ipc_table));
+        ipc_table.data = get_ipc(table);
+
+        rc = proc_dointvec(&ipc_table, write, filp, buffer, lenp, ppos);
 
         if (write && !rc && lenp_bef == *lenp)
-                /*
-                 * Tunable has successfully been changed from userland:
-                 * disable its automatic recomputing.
-                 */
-                unregister_ipcns_notifier(current->nsproxy->ipc_ns);
+                tunable_set_callback(*((int *)(ipc_table.data)));
 
         return rc;
 }
@@ -119,12 +142,14 @@ static int sysctl_ipc_registered_data(ctl_table *table, int __user *name,
         rc = sysctl_ipc_data(table, name, nlen, oldval, oldlenp, newval,
                 newlen);
 
-        if (newval && newlen && rc > 0)
+        if (newval && newlen && rc > 0) {
                 /*
-                 * Tunable has successfully been changed from userland:
-                 * disable its automatic recomputing.
+                 * Tunable has successfully been changed from userland
                  */
-                unregister_ipcns_notifier(current->nsproxy->ipc_ns);
+                int *data = get_ipc(table);
+
+                tunable_set_callback(*data);
+        }
 
         return rc;
 }
diff --git a/ipc/ipcns_notifier.c b/ipc/ipcns_notifier.c
index c7974609defc..70ff09183f7b 100644
--- a/ipc/ipcns_notifier.c
+++ b/ipc/ipcns_notifier.c
@@ -61,6 +61,15 @@ int register_ipcns_notifier(struct ipc_namespace *ns)
         return blocking_notifier_chain_register(&ipcns_chain, &ns->ipcns_nb);
 }
 
+int cond_register_ipcns_notifier(struct ipc_namespace *ns)
+{
+        memset(&ns->ipcns_nb, 0, sizeof(ns->ipcns_nb));
+        ns->ipcns_nb.notifier_call = ipcns_callback;
+        ns->ipcns_nb.priority = IPCNS_CALLBACK_PRI;
+        return blocking_notifier_chain_cond_register(&ipcns_chain,
+                                                        &ns->ipcns_nb);
+}
+
 int unregister_ipcns_notifier(struct ipc_namespace *ns)
 {
         return blocking_notifier_chain_unregister(&ipcns_chain,
diff --git a/kernel/notifier.c b/kernel/notifier.c
index 643360d1bb14..823be11584ef 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -31,6 +31,21 @@ static int notifier_chain_register(struct notifier_block **nl,
         return 0;
 }
 
+static int notifier_chain_cond_register(struct notifier_block **nl,
+                struct notifier_block *n)
+{
+        while ((*nl) != NULL) {
+                if ((*nl) == n)
+                        return 0;
+                if (n->priority > (*nl)->priority)
+                        break;
+                nl = &((*nl)->next);
+        }
+        n->next = *nl;
+        rcu_assign_pointer(*nl, n);
+        return 0;
+}
+
 static int notifier_chain_unregister(struct notifier_block **nl,
                 struct notifier_block *n)
 {
@@ -205,6 +220,29 @@ int blocking_notifier_chain_register(struct blocking_notifier_head *nh,
 EXPORT_SYMBOL_GPL(blocking_notifier_chain_register);
 
 /**
+ *      blocking_notifier_chain_cond_register - Cond add notifier to a blocking notifier chain
+ *      @nh: Pointer to head of the blocking notifier chain
+ *      @n: New entry in notifier chain
+ *
+ *      Adds a notifier to a blocking notifier chain, only if not already
+ *      present in the chain.
+ *      Must be called in process context.
+ *
+ *      Currently always returns zero.
+ */
+int blocking_notifier_chain_cond_register(struct blocking_notifier_head *nh,
+                struct notifier_block *n)
+{
+        int ret;
+
+        down_write(&nh->rwsem);
+        ret = notifier_chain_cond_register(&nh->head, n);
+        up_write(&nh->rwsem);
+        return ret;
+}
+EXPORT_SYMBOL_GPL(blocking_notifier_chain_cond_register);
+
+/**
  *      blocking_notifier_chain_unregister - Remove notifier from a blocking notifier chain
  *      @nh: Pointer to head of the blocking notifier chain
  *      @n: Entry to remove from notifier chain