On Tue, 26 Mar 2013 14:07:14 -0400
Sasha Levin <sasha.le...@oracle.com> wrote:
> > Not necessarily, we do release everything at the end of the function:
> > out_unlock_free:
> > sem_unlock(sma, locknum);
>
> Ow, there's a rcu_read_unlock() in sem_unlock()? This complicates things even
> more I suspect. If un is non-NULL we'll be unlocking rcu lock twice?
Sasha, this patch should resolve the RCU tangle, by making sure
we only ever take the rcu_read_lock once in semtimedop.
---8<---
The ipc semaphore code has a nasty RCU locking tangle, with both
find_alloc_undo and semtimedop taking the rcu_read_lock(). The
code can be cleaned up somewhat by only taking the rcu_read_lock
once.
There are no other callers to find_alloc_undo.
This should also solve the trinity issue reported by Sasha Levin.
Reported-by: Sasha Levin <sasha.le...@oracle.com>
Signed-off-by: Rik van Riel <r...@redhat.com>
---
ipc/sem.c | 31 +++++++++----------------------
1 files changed, 9 insertions(+), 22 deletions(-)
diff --git a/ipc/sem.c b/ipc/sem.c
index f46441a..2ec2945 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1646,22 +1646,23 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf
__user *, tsops,
alter = 1;
}
+ INIT_LIST_HEAD(&tasks);
+
if (undos) {
+ /* On success, find_alloc_undo takes the rcu_read_lock */
un = find_alloc_undo(ns, semid);
if (IS_ERR(un)) {
error = PTR_ERR(un);
goto out_free;
}
- } else
+ } else {
un = NULL;
+ rcu_read_lock();
+ }
- INIT_LIST_HEAD(&tasks);
-
- rcu_read_lock();
sma = sem_obtain_object_check(ns, semid);
if (IS_ERR(sma)) {
- if (un)
- rcu_read_unlock();
+ rcu_read_unlock();
error = PTR_ERR(sma);
goto out_free;
}
@@ -1693,22 +1694,8 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf
__user *, tsops,
*/
error = -EIDRM;
locknum = sem_lock(sma, sops, nsops);
- 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();
- }
- }
+ if (un && un->semid == -1)
+ goto out_unlock_free;
error = try_atomic_semop (sma, sops, nsops, un, task_tgid_vnr(current));
if (error <= 0) {
--
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