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>

1 files changed, 5 insertions, 32 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index a82c88d0900f..e08b94851922 100644
--- 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.