ipc/sem: avoid idr tree lookup for interrupted semop

We can avoid the idr tree lookup (albeit possibly avoiding
idr_find_fast()) when being awoken in EINTR, as the semid will not
change in this context while blocked.  Use the sma pointer directly and
take the sem_lock, then re-check for RMID races.  We continue to
re-check the queue.status with the lock held such that we can detect
situations where we where are dealing with a spurious wakeup but another
task that holds the sem_lock updated the queue.status while we were
spinning for it.  Once we take the lock it obviously won't change again.

Being the only caller, get rid of sem_obtain_lock() altogether.

Link: http://lkml.kernel.org/r/1478708774-28826-3-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/ipc/sem.c b/ipc/sem.c
index a82c88d0900f307435985d1ee3504f29188ed2c5..e08b948519223e62aca265284483ee7b1ef1420c 100644 (file)
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -414,29 +414,6 @@ static inline void sem_unlock(struct sem_array *sma, int locknum)
  *
  * The caller holds the RCU read lock.
  */
-static inline struct sem_array *sem_obtain_lock(struct ipc_namespace *ns,
-                       int id, struct sembuf *sops, int nsops, int *locknum)
-{
-       struct kern_ipc_perm *ipcp;
-       struct sem_array *sma;
-
-       ipcp = ipc_obtain_object_idr(&sem_ids(ns), id);
-       if (IS_ERR(ipcp))
-               return ERR_CAST(ipcp);
-
-       sma = container_of(ipcp, struct sem_array, sem_perm);
-       *locknum = sem_lock(sma, sops, nsops);
-
-       /* ipc_rmid() may have already freed the ID while sem_lock
-        * was spinning: verify that the structure is still valid
-        */
-       if (ipc_valid_object(ipcp))
-               return container_of(ipcp, struct sem_array, sem_perm);
-
-       sem_unlock(sma, *locknum);
-       return ERR_PTR(-EINVAL);
-}
-
 static inline struct sem_array *sem_obtain_object(struct ipc_namespace *ns, int id)
 {
        struct kern_ipc_perm *ipcp = ipc_obtain_object_idr(&sem_ids(ns), id);
@@ -2000,16 +1977,12 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
                }
 
                rcu_read_lock();
-               sma = sem_obtain_lock(ns, semid, sops, nsops, &locknum);
-               error = READ_ONCE(queue.status);
+               sem_lock(sma, sops, nsops);
 
-               /*
-                * Array removed? If yes, leave without sem_unlock().
-                */
-               if (IS_ERR(sma)) {
-                       rcu_read_unlock();
-                       goto out_free;
-               }
+               if (!ipc_valid_object(&sma->sem_perm))
+                       goto out_unlock_free;
+
+               error = READ_ONCE(queue.status);
 
                /*
                 * If queue.status != -EINTR we are woken up by another process.