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Diffstat (limited to 'ipc/sem.c')
-rw-r--r--ipc/sem.c47
1 files changed, 35 insertions, 12 deletions
diff --git a/ipc/sem.c b/ipc/sem.c
index bc3d530cb23e..b471e5a3863d 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -253,6 +253,16 @@ static void sem_rcu_free(struct rcu_head *head)
}
/*
+ * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they
+ * are only control barriers.
+ * The code must pair with spin_unlock(&sem->lock) or
+ * spin_unlock(&sem_perm.lock), thus just the control barrier is insufficient.
+ *
+ * smp_rmb() is sufficient, as writes cannot pass the control barrier.
+ */
+#define ipc_smp_acquire__after_spin_is_unlocked() smp_rmb()
+
+/*
* Wait until all currently ongoing simple ops have completed.
* Caller must own sem_perm.lock.
* New simple ops cannot start, because simple ops first check
@@ -275,6 +285,7 @@ static void sem_wait_array(struct sem_array *sma)
sem = sma->sem_base + i;
spin_unlock_wait(&sem->lock);
}
+ ipc_smp_acquire__after_spin_is_unlocked();
}
/*
@@ -327,13 +338,12 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
/* Then check that the global lock is free */
if (!spin_is_locked(&sma->sem_perm.lock)) {
/*
- * The ipc object lock check must be visible on all
- * cores before rechecking the complex count. Otherwise
- * we can race with another thread that does:
+ * We need a memory barrier with acquire semantics,
+ * otherwise we can race with another thread that does:
* complex_count++;
* spin_unlock(sem_perm.lock);
*/
- smp_rmb();
+ ipc_smp_acquire__after_spin_is_unlocked();
/*
* Now repeat the test of complex_count:
@@ -2074,17 +2084,28 @@ void exit_sem(struct task_struct *tsk)
rcu_read_lock();
un = list_entry_rcu(ulp->list_proc.next,
struct sem_undo, list_proc);
- if (&un->list_proc == &ulp->list_proc)
- semid = -1;
- else
- semid = un->semid;
+ if (&un->list_proc == &ulp->list_proc) {
+ /*
+ * We must wait for freeary() before freeing this ulp,
+ * in case we raced with last sem_undo. There is a small
+ * possibility where we exit while freeary() didn't
+ * finish unlocking sem_undo_list.
+ */
+ spin_unlock_wait(&ulp->lock);
+ rcu_read_unlock();
+ break;
+ }
+ spin_lock(&ulp->lock);
+ semid = un->semid;
+ spin_unlock(&ulp->lock);
+ /* exit_sem raced with IPC_RMID, nothing to do */
if (semid == -1) {
rcu_read_unlock();
- break;
+ continue;
}
- sma = sem_obtain_object_check(tsk->nsproxy->ipc_ns, un->semid);
+ sma = sem_obtain_object_check(tsk->nsproxy->ipc_ns, semid);
/* exit_sem raced with IPC_RMID, nothing to do */
if (IS_ERR(sma)) {
rcu_read_unlock();
@@ -2112,9 +2133,11 @@ void exit_sem(struct task_struct *tsk)
ipc_assert_locked_object(&sma->sem_perm);
list_del(&un->list_id);
- spin_lock(&ulp->lock);
+ /* we are the last process using this ulp, acquiring ulp->lock
+ * isn't required. Besides that, we are also protected against
+ * IPC_RMID as we hold sma->sem_perm lock now
+ */
list_del_rcu(&un->list_proc);
- spin_unlock(&ulp->lock);
/* perform adjustments registered in un */
for (i = 0; i < sma->sem_nsems; i++) {