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 <[EMAIL PROTECTED]>
Signed-off-by: Nadia Derbey <[EMAIL PROTECTED]>
---
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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