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From: pierre.peiffer@bull.net
To: linux-kernel@vger.kernel.org, containers@lists.linux-foundation.org
Subject: [PATCH 2.6.24-rc8-mm1 12/15] (RFC) IPC/semaphores: make use of RCU to free the sem_undo_list
Date: Tue, 29 Jan 2008 17:02:41 +0100	[thread overview]
Message-ID: <20080129162131.743970100@bull.net> (raw)
In-Reply-To: <20080129160229.612172683@bull.net>

[-- Attachment #1: ipc_exit_sem_use_rcu.patch --]
[-- Type: text/plain, Size: 5080 bytes --]

From: Pierre Peiffer <pierre.peiffer@bull.net>

Today, the sem_undo_list is freed when the last task using it exits.
There is no mechanism in place, that allows a safe concurrent access to
the sem_undo_list of a target task and protects efficiently against a
task-exit.

That is okay for now as we don't need this.

As I would like to provide a /proc interface to access this data, I need
such a safe access, without blocking the target task if possible. 

This patch proposes to introduce the use of RCU to delay the real free of
these sem_undo_list structures. They can then be accessed in a safe manner
by any tasks inside read critical section, this way:

	struct sem_undo_list *undo_list;
	int ret;
	...
	rcu_read_lock();
	undo_list = rcu_dereference(task->sysvsem.undo_list);
	if (undo_list)
		ret = atomic_inc_not_zero(&undo_list->refcnt);
	rcu_read_unlock();
	...
	if (undo_list && ret) {
		/* section where undo_list can be used quietly */
		...
	}
	...

Signed-off-by: Pierre Peiffer <pierre.peiffer@bull.net>
---

 include/linux/sem.h |    7 +++++--
 ipc/sem.c           |   42 ++++++++++++++++++++++++++----------------
 2 files changed, 31 insertions(+), 18 deletions(-)

Index: b/include/linux/sem.h
===================================================================
--- a/include/linux/sem.h
+++ b/include/linux/sem.h
@@ -115,7 +115,8 @@ struct sem_queue {
 };
 
 /* Each task has a list of undo requests. They are executed automatically
- * when the process exits.
+ * when the last refcnt of sem_undo_list is released (ie when the process exits
+ * in the general case)
  */
 struct sem_undo {
 	struct sem_undo *	proc_next;	/* next entry on this process */
@@ -125,12 +126,14 @@ struct sem_undo {
 };
 
 /* sem_undo_list controls shared access to the list of sem_undo structures
- * that may be shared among all a CLONE_SYSVSEM task group.
+ * that may be shared among all a CLONE_SYSVSEM task group or with an external
+ * process which changes the list through procfs.
  */ 
 struct sem_undo_list {
 	atomic_t	refcnt;
 	spinlock_t	lock;
 	struct sem_undo	*proc_list;
+	struct ipc_namespace *ns;
 };
 
 struct sysv_sem {
Index: b/ipc/sem.c
===================================================================
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1038,6 +1038,7 @@ static inline int get_undo_list(struct s
 			return -ENOMEM;
 		spin_lock_init(&undo_list->lock);
 		atomic_set(&undo_list->refcnt, 1);
+		undo_list->ns = get_ipc_ns(current->nsproxy->ipc_ns);
 		current->sysvsem.undo_list = undo_list;
 	}
 	*undo_listp = undo_list;
@@ -1316,7 +1317,8 @@ int copy_semundo(unsigned long clone_fla
 }
 
 /*
- * add semadj values to semaphores, free undo structures.
+ * add semadj values to semaphores, free undo structures, if there is no
+ * more user.
  * undo structures are not freed when semaphore arrays are destroyed
  * so some of them may be out of date.
  * IMPLEMENTATION NOTE: There is some confusion over whether the
@@ -1326,23 +1328,17 @@ int copy_semundo(unsigned long clone_fla
  * The original implementation attempted to do this (queue and wait).
  * The current implementation does not do so. The POSIX standard
  * and SVID should be consulted to determine what behavior is mandated.
+ *
+ * Note:
+ * A concurrent task is only allowed to access and go through the list
+ * of sem_undo if it successfully grabs a refcnt.
  */
-void exit_sem(struct task_struct *tsk)
+static void free_semundo_list(struct sem_undo_list *undo_list)
 {
-	struct sem_undo_list *undo_list;
 	struct sem_undo *u, **up;
-	struct ipc_namespace *ns;
 
-	undo_list = tsk->sysvsem.undo_list;
-	if (!undo_list)
-		return;
-
-	if (!atomic_dec_and_test(&undo_list->refcnt))
-		return;
-
-	ns = tsk->nsproxy->ipc_ns;
-	/* There's no need to hold the semundo list lock, as current
-         * is the last task exiting for this undo list.
+	/* There's no need to hold the semundo list lock, as there are
+	 * no more tasks or possible users for this undo list.
 	 */
 	for (up = &undo_list->proc_list; (u = *up); *up = u->proc_next, kfree(u)) {
 		struct sem_array *sma;
@@ -1354,7 +1350,7 @@ void exit_sem(struct task_struct *tsk)
 
 		if(semid == -1)
 			continue;
-		sma = sem_lock(ns, semid);
+		sma = sem_lock(undo_list->ns, semid);
 		if (IS_ERR(sma))
 			continue;
 
@@ -1368,7 +1364,8 @@ void exit_sem(struct task_struct *tsk)
 			if (u == un)
 				goto found;
 		}
-		printk ("exit_sem undo list error id=%d\n", u->semid);
+		printk(KERN_ERR "free_semundo_list error id=%d\n",
+		       u->semid);
 		goto next_entry;
 found:
 		*unp = un->id_next;
@@ -1404,9 +1401,22 @@ found:
 next_entry:
 		sem_unlock(sma);
 	}
+	put_ipc_ns(undo_list->ns);
 	kfree(undo_list);
 }
 
+/* called from do_exit() */
+void exit_sem(struct task_struct *tsk)
+{
+	struct sem_undo_list *ul = tsk->sysvsem.undo_list;
+	if (ul) {
+		rcu_assign_pointer(tsk->sysvsem.undo_list, NULL);
+		synchronize_rcu();
+		if (atomic_dec_and_test(&ul->refcnt))
+			free_semundo_list(ul);
+	}
+}
+
 #ifdef CONFIG_PROC_FS
 static int sysvipc_sem_proc_show(struct seq_file *s, void *it)
 {

-- 
Pierre Peiffer

  parent reply	other threads:[~2008-01-29 16:23 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-29 16:02 [PATCH 2.6.24-rc8-mm1 00/15] IPC: code rewrite + new functionalities pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 01/15] IPC/semaphores: code factorisation pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 02/15] IPC/shared memory: introduce shmctl_down pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 03/15] IPC/message queues: introduce msgctl_down pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 04/15] IPC/semaphores: move the rwmutex handling inside semctl_down pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 05/15] IPC/semaphores: remove one unused parameter from semctl_down() pierre.peiffer
2008-01-31  8:32   ` Nadia Derbey
2008-01-31 10:18     ` Pierre Peiffer
2008-01-31 11:30       ` Nadia Derbey
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 06/15] IPC: get rid of the use *_setbuf structure pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 07/15] IPC: introduce ipc_update_perm() pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 08/15] IPC: consolidate all xxxctl_down() functions pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 09/15] (RFC) IPC: new kernel API to change an ID pierre.peiffer
2008-01-29 21:06   ` Alexey Dobriyan
2008-01-30  9:52     ` Pierre Peiffer
2008-01-31  9:00     ` Pierre Peiffer
2008-01-31  9:54       ` Kirill Korotaev
2008-01-31 11:57         ` Pierre Peiffer
2008-01-31 13:11           ` Kirill Korotaev
2008-01-31 16:10             ` Cedric Le Goater
2008-02-04 13:41               ` Kirill Korotaev
2008-02-04 14:06                 ` [Devel] " Pavel Emelyanov
2008-02-04 15:00                   ` Daniel Lezcano
2008-02-04 15:16                     ` Pavel Emelyanov
2008-02-05  9:51           ` Oren Laadan
2008-02-05 18:00             ` Dave Hansen
2008-02-05 18:42             ` Serge E. Hallyn
2008-02-06  2:07               ` Oren Laadan
2008-02-06  5:00                 ` Serge E. Hallyn
2008-02-08 10:12               ` Pierre Peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 10/15] (RFC) IPC: new IPC_SETID command to modify " pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 11/15] (RFC) IPC: new IPC_SETALL command to modify all settings pierre.peiffer
2008-01-29 16:02 ` pierre.peiffer [this message]
2008-01-30 21:26   ` [PATCH 2.6.24-rc8-mm1 12/15] (RFC) IPC/semaphores: make use of RCU to free the sem_undo_list Serge E. Hallyn
2008-01-31  9:52     ` Pierre Peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 13/15] (RFC) IPC/semaphores: per <pid> semundo file in procfs pierre.peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 14/15] (RFC) IPC/semaphores: prepare semundo code to work on another task than current pierre.peiffer
2008-01-30 21:44   ` Serge E. Hallyn
2008-01-31  9:48     ` Pierre Peiffer
2008-01-31 18:01       ` Serge E. Hallyn
2008-02-01 12:09         ` Pierre Peiffer
2008-01-29 16:02 ` [PATCH 2.6.24-rc8-mm1 15/15] (RFC) IPC/semaphores: add write() operation to semundo file in procfs pierre.peiffer
2008-02-02 18:23 ` [PATCH 2.6.24-rc8-mm1 00/15] IPC: code rewrite + new functionalities Pavel Machek
2008-02-04 13:52   ` Pierre Peiffer
2008-02-04 15:44   ` Benjamin Thery
2008-02-04 19:51     ` Pavel Machek

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