ipc/sem.c: rewrite undo list locking
The attached patch:
- reverses the locking order of ulp->lock and sem_lock:
Previously, it was first ulp->lock, then inside sem_lock.
Now it's the other way around.
- converts the undo structure to rcu.
Benefits:
- With the old locking order, IPC_RMID could not kfree the undo structures.
The stale entries remained in the linked lists and were released later.
- The patch fixes a a race in semtimedop(): if both IPC_RMID and a semget() that
recreates exactly the same id happen between find_alloc_undo() and sem_lock,
then semtimedop() would access already kfree'd memory.
[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Reviewed-by: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: Pierre Peiffer <peifferp@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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 @@
#ifdef __KERNEL__
#include <asm/atomic.h>
+#include <linux/rcupdate.h>
struct task_struct;
@@ -114,7 +115,10 @@
* 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 */
diff --git a/ipc/sem.c b/ipc/sem.c
index 3ca2327..bf1bc36 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -504,27 +504,35 @@
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 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 @@
* 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 @@
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 @@
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);
-
- sem_unlock(sma);
- spin_unlock(&ulp->lock);
un = new;
+
+success:
+ spin_unlock(&ulp->lock);
+ rcu_read_lock();
+ sem_unlock(sma);
out:
return un;
}
@@ -1103,6 +1110,8 @@
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 @@
* 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 @@
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 @@
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 (semid == -1)
+ break;
+
+ sma = sem_lock_check(tsk->nsproxy->ipc_ns, un->semid);
+
+ /* exit_sem raced with IPC_RMID, nothing to do */
if (IS_ERR(sma))
- goto free;
+ continue;
- if (un->semid == -1)
- goto unlock_free;
+ 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;
+ }
- BUG_ON(sem_checkid(sma, un->semid));
-
- /* remove un from sma->list_id */
+ /* 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 @@
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);
}
