diff options
| -rw-r--r-- | include/linux/sem.h | 6 | ||||
| -rw-r--r-- | ipc/sem.c | 147 |
2 files changed, 98 insertions, 55 deletions
diff --git a/include/linux/sem.h b/include/linux/sem.h index d425993..1b191c1 100644 --- a/include/linux/sem.h +++ b/include/linux/sem.h @@ -78,6 +78,7 @@ struct seminfo { #ifdef __KERNEL__ #include <asm/atomic.h> +#include <linux/rcupdate.h> struct task_struct; @@ -114,7 +115,10 @@ struct sem_queue { * when the process exits. */ struct sem_undo { - struct list_head list_proc; /* per-process list: all undos from one process */ + struct list_head list_proc; /* per-process list: all undos from one process. */ + /* rcu protected */ + struct rcu_head rcu; /* rcu struct for sem_undo() */ + struct sem_undo_list *ulp; /* sem_undo_list for the process */ struct list_head list_id; /* per semaphore array list: all undos for one array */ int semid; /* semaphore set identifier */ short * semadj; /* array of adjustments, one per semaphore */ @@ -504,27 +504,35 @@ static int count_semzcnt (struct sem_array * sma, ushort semnum) return semzcnt; } +void free_un(struct rcu_head *head) +{ + struct sem_undo *un = container_of(head, struct sem_undo, rcu); + kfree(un); +} + /* Free a semaphore set. freeary() is called with sem_ids.rw_mutex locked * as a writer and the spinlock for this semaphore set hold. sem_ids.rw_mutex * remains locked on exit. */ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp) { - struct sem_undo *un; - struct sem_queue *q, *t; + struct sem_undo *un, *tu; + struct sem_queue *q, *tq; struct sem_array *sma = container_of(ipcp, struct sem_array, sem_perm); - /* Invalidate the existing undo structures for this semaphore set. - * (They will be freed without any further action in exit_sem() - * or during the next semop.) - */ + /* Free the existing undo structures for this semaphore set. */ assert_spin_locked(&sma->sem_perm.lock); - list_for_each_entry(un, &sma->list_id, list_id) + list_for_each_entry_safe(un, tu, &sma->list_id, list_id) { + list_del(&un->list_id); + spin_lock(&un->ulp->lock); un->semid = -1; + list_del_rcu(&un->list_proc); + spin_unlock(&un->ulp->lock); + call_rcu(&un->rcu, free_un); + } /* Wake up all pending processes and let them fail with EIDRM. */ - - list_for_each_entry_safe(q, t, &sma->sem_pending, list) { + list_for_each_entry_safe(q, tq, &sma->sem_pending, list) { list_del(&q->list); q->status = IN_WAKEUP; @@ -948,16 +956,11 @@ static inline int get_undo_list(struct sem_undo_list **undo_listp) static struct sem_undo *lookup_undo(struct sem_undo_list *ulp, int semid) { - struct sem_undo *walk, *tmp; + struct sem_undo *walk; - assert_spin_locked(&ulp->lock); - list_for_each_entry_safe(walk, tmp, &ulp->list_proc, list_proc) { + list_for_each_entry_rcu(walk, &ulp->list_proc, list_proc) { if (walk->semid == semid) return walk; - if (walk->semid == -1) { - list_del(&walk->list_proc); - kfree(walk); - } } return NULL; } @@ -970,6 +973,8 @@ static struct sem_undo *lookup_undo(struct sem_undo_list *ulp, int semid) * The function looks up (and if not present creates) the undo structure. * The size of the undo structure depends on the size of the semaphore * array, thus the alloc path is not that straightforward. + * Lifetime-rules: sem_undo is rcu-protected, on success, the function + * performs a rcu_read_lock(). */ static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid) { @@ -983,11 +988,13 @@ static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid) if (error) return ERR_PTR(error); + rcu_read_lock(); spin_lock(&ulp->lock); un = lookup_undo(ulp, semid); spin_unlock(&ulp->lock); if (likely(un!=NULL)) goto out; + rcu_read_unlock(); /* no undo structure around - allocate one. */ /* step 1: figure out the size of the semaphore array */ @@ -1005,38 +1012,38 @@ static struct sem_undo *find_alloc_undo(struct ipc_namespace *ns, int semid) return ERR_PTR(-ENOMEM); } - /* step 3: Acquire the lock on the undo list pointer */ - spin_lock(&ulp->lock); - - /* step 4: check for races: someone else allocated the undo struct, - * semaphore array was destroyed. - */ - un = lookup_undo(ulp, semid); - if (un) { - spin_unlock(&ulp->lock); - kfree(new); - sem_putref(sma); - goto out; - } + /* step 3: Acquire the lock on semaphore array */ sem_lock_and_putref(sma); if (sma->sem_perm.deleted) { sem_unlock(sma); - spin_unlock(&ulp->lock); kfree(new); un = ERR_PTR(-EIDRM); goto out; } + spin_lock(&ulp->lock); + + /* + * step 4: check for races: did someone else allocate the undo struct? + */ + un = lookup_undo(ulp, semid); + if (un) { + kfree(new); + goto success; + } /* step 5: initialize & link new undo structure */ new->semadj = (short *) &new[1]; + new->ulp = ulp; new->semid = semid; assert_spin_locked(&ulp->lock); - list_add(&new->list_proc, &ulp->list_proc); + list_add_rcu(&new->list_proc, &ulp->list_proc); assert_spin_locked(&sma->sem_perm.lock); list_add(&new->list_id, &sma->list_id); + un = new; - sem_unlock(sma); +success: spin_unlock(&ulp->lock); - un = new; + rcu_read_lock(); + sem_unlock(sma); out: return un; } @@ -1103,6 +1110,8 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops, sma = sem_lock_check(ns, semid); if (IS_ERR(sma)) { + if (un) + rcu_read_unlock(); error = PTR_ERR(sma); goto out_free; } @@ -1111,10 +1120,26 @@ asmlinkage long sys_semtimedop(int semid, struct sembuf __user *tsops, * semid identifiers are not unique - find_alloc_undo may have * allocated an undo structure, it was invalidated by an RMID * and now a new array with received the same id. Check and fail. + * This case can be detected checking un->semid. The existance of + * "un" itself is guaranteed by rcu. */ error = -EIDRM; - if (un && un->semid == -1) - goto out_unlock_free; + if (un) { + if (un->semid == -1) { + rcu_read_unlock(); + goto out_unlock_free; + } else { + /* + * rcu lock can be released, "un" cannot disappear: + * - sem_lock is acquired, thus IPC_RMID is + * impossible. + * - exit_sem is impossible, it always operates on + * current (or a dead task). + */ + + rcu_read_unlock(); + } + } error = -EFBIG; if (max >= sma->sem_nsems) @@ -1242,7 +1267,6 @@ int copy_semundo(unsigned long clone_flags, struct task_struct *tsk) void exit_sem(struct task_struct *tsk) { struct sem_undo_list *ulp; - struct sem_undo *un, *tmp; ulp = tsk->sysvsem.undo_list; if (!ulp) @@ -1252,28 +1276,47 @@ void exit_sem(struct task_struct *tsk) if (!atomic_dec_and_test(&ulp->refcnt)) return; - spin_lock(&ulp->lock); - - list_for_each_entry_safe(un, tmp, &ulp->list_proc, list_proc) { + for (;;) { struct sem_array *sma; + struct sem_undo *un; + int semid; int i; - if (un->semid == -1) - goto free; + rcu_read_lock(); + un = list_entry(rcu_dereference(ulp->list_proc.next), + struct sem_undo, list_proc); + if (&un->list_proc == &ulp->list_proc) + semid = -1; + else + semid = un->semid; + rcu_read_unlock(); - sma = sem_lock(tsk->nsproxy->ipc_ns, un->semid); - if (IS_ERR(sma)) - goto free; + if (semid == -1) + break; - if (un->semid == -1) - goto unlock_free; + sma = sem_lock_check(tsk->nsproxy->ipc_ns, un->semid); - BUG_ON(sem_checkid(sma, un->semid)); + /* exit_sem raced with IPC_RMID, nothing to do */ + if (IS_ERR(sma)) + continue; - /* remove un from sma->list_id */ + un = lookup_undo(ulp, semid); + if (un == NULL) { + /* exit_sem raced with IPC_RMID+semget() that created + * exactly the same semid. Nothing to do. + */ + sem_unlock(sma); + continue; + } + + /* remove un from the linked lists */ assert_spin_locked(&sma->sem_perm.lock); list_del(&un->list_id); + spin_lock(&ulp->lock); + list_del_rcu(&un->list_proc); + spin_unlock(&ulp->lock); + /* perform adjustments registered in un */ for (i = 0; i < sma->sem_nsems; i++) { struct sem * semaphore = &sma->sem_base[i]; @@ -1302,14 +1345,10 @@ void exit_sem(struct task_struct *tsk) sma->sem_otime = get_seconds(); /* maybe some queued-up processes were waiting for this */ update_queue(sma); -unlock_free: sem_unlock(sma); -free: - assert_spin_locked(&ulp->lock); - list_del(&un->list_proc); - kfree(un); + + call_rcu(&un->rcu, free_un); } - spin_unlock(&ulp->lock); kfree(ulp); } |
