Quoting [EMAIL PROTECTED] ([EMAIL PROTECTED]):
> 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]>
Acked-by: Serge Hallyn <[EMAIL PROTECTED]>
thanks,
-serge
>
> ---
> 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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