[Devel] [RFC PATCH 1/6] IPC/sem: RCU-protect the process semundo list

[Nadia.Derbey at bull.net]

PATCH [01/06]

Today, 'current' has an exclusive access to its sem_undo_list (anchored at
current->sysvsem.undo_list):
  . it is created during a semop() if the SEMUNDO flag is specified for one
    of the semaphores.
  . it can also be created during a copy_process() operation if the
    CLONE_SYSVSEM flag is specified (in that case the undo_list is created/
    copied from 'current' into the target task but that target task is not
    running yet).
  . it is freed during an unshare() or an exit() operation, if the caller
    (current) is the last task using it.


In order to allow a third party process to read a process' undo list, without
a too big performance impact on the existing operations, this patch proposes
to make the sem_undo_list structures rcu protected.

They can then be safely accessed by any task 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 at bull.net>
Signed-off-by: Nadia Derbey <Nadia.Derbey at bull.net>

---
 include/linux/sem.h |    4 +++-
 ipc/sem.c           |   22 ++++++++++++++++++----
 2 files changed, 21 insertions(+), 5 deletions(-)

Index: linux-2.6.26-rc5-mm3/include/linux/sem.h
===================================================================
--- linux-2.6.26-rc5-mm3.orig/include/linux/sem.h	2008-06-24 09:04:21.000000000 +0200
+++ linux-2.6.26-rc5-mm3/include/linux/sem.h	2008-06-24 10:01:50.000000000 +0200
@@ -112,7 +112,8 @@ 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 list_head	list_proc;	/* per-process list: all undos from one process. */
@@ -131,6 +132,7 @@ struct sem_undo_list {
 	atomic_t		refcnt;
 	spinlock_t		lock;
 	struct list_head	list_proc;
+	struct rcu_head		rcu;
 };
 
 struct sysv_sem {
Index: linux-2.6.26-rc5-mm3/ipc/sem.c
===================================================================
--- linux-2.6.26-rc5-mm3.orig/ipc/sem.c	2008-06-24 09:05:03.000000000 +0200
+++ linux-2.6.26-rc5-mm3/ipc/sem.c	2008-06-24 09:37:33.000000000 +0200
@@ -939,6 +939,10 @@ static inline int get_undo_list(struct s
 {
 	struct sem_undo_list *undo_list;
 
+	/*
+	 * No need to have a rcu read critical section here: no one but
+	 * current is accessing the undo_list.
+	 */
 	undo_list = current->sysvsem.undo_list;
 	if (!undo_list) {
 		undo_list = kzalloc(sizeof(*undo_list), GFP_KERNEL);
@@ -948,7 +952,7 @@ static inline int get_undo_list(struct s
 		atomic_set(&undo_list->refcnt, 1);
 		INIT_LIST_HEAD(&undo_list->list_proc);
 
-		current->sysvsem.undo_list = undo_list;
+		rcu_assign_pointer(current->sysvsem.undo_list, undo_list);
 	}
 	*undo_listp = undo_list;
 	return 0;
@@ -1268,10 +1272,15 @@ void exit_sem(struct task_struct *tsk)
 {
 	struct sem_undo_list *ulp;
 
-	ulp = tsk->sysvsem.undo_list;
-	if (!ulp)
+	rcu_read_lock();
+	ulp = rcu_dereference(tsk->sysvsem.undo_list);
+	if (!ulp) {
+		rcu_read_unlock();
 		return;
-	tsk->sysvsem.undo_list = NULL;
+	}
+	rcu_read_unlock();
+	rcu_assign_pointer(tsk->sysvsem.undo_list, NULL);
+	synchronize_rcu();
 
 	if (!atomic_dec_and_test(&ulp->refcnt))
 		return;
@@ -1349,6 +1358,11 @@ void exit_sem(struct task_struct *tsk)
 
 		call_rcu(&un->rcu, free_un);
 	}
+	/*
+	 * No need to call synchronize_rcu() here: we come here if the refcnt
+	 * is 0 and this has been done into exit_sem after synchronizing. So
+	 * nobody else can be referencing to the undo_list.
+	 */
 	kfree(ulp);
 }
 

--
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