ipc/sem: rework task wakeups

Our sysv sems have been using the notion of lockless wakeups for a
while, ever since commit 0a2b9d4c7967 ("ipc/sem.c: move wake_up_process
out of the spinlock section"), in order to reduce the sem_lock hold
times.  This in-house pending queue can be replaced by wake_q (just like
all the rest of ipc now), in that it provides the following advantages:

 o Simplifies and gets rid of unnecessary code.

 o We get rid of the IN_WAKEUP complexities. Given that wake_q_add()
   grabs reference to the task, if awoken due to an unrelated event,
   between the wake_q_add() and wake_up_q() window, we cannot race with
   sys_exit and the imminent call to wake_up_process().

 o By not spinning IN_WAKEUP, we no longer need to disable preemption.

In consequence, the wakeup paths (after schedule(), that is) must
acknowledge an external signal/event, as well spurious wakeup occurring
during the pending wakeup window.  Obviously no changes in semantics
that could be visible to the user.  The fastpath is _only_ for when we
know for sure that we were awoken due to a the waker's successful semop
call (queue.status is not -EINTR).

On a 48-core Haswell, running the ipcscale 'waitforzero' test, the
following is seen with increasing thread counts:

                               v4.8-rc5                v4.8-rc5
                                                        semopv2
Hmean    sembench-sem-2      574733.00 (  0.00%)   578322.00 (  0.62%)
Hmean    sembench-sem-8      811708.00 (  0.00%)   824689.00 (  1.59%)
Hmean    sembench-sem-12     842448.00 (  0.00%)   845409.00 (  0.35%)
Hmean    sembench-sem-21     933003.00 (  0.00%)   977748.00 (  4.80%)
Hmean    sembench-sem-48     935910.00 (  0.00%)  1004759.00 (  7.36%)
Hmean    sembench-sem-79     937186.00 (  0.00%)   983976.00 (  4.99%)
Hmean    sembench-sem-234    974256.00 (  0.00%)  1060294.00 (  8.83%)
Hmean    sembench-sem-265    975468.00 (  0.00%)  1016243.00 (  4.18%)
Hmean    sembench-sem-296    991280.00 (  0.00%)  1042659.00 (  5.18%)
Hmean    sembench-sem-327    975415.00 (  0.00%)  1029977.00 (  5.59%)
Hmean    sembench-sem-358   1014286.00 (  0.00%)  1049624.00 (  3.48%)
Hmean    sembench-sem-389    972939.00 (  0.00%)  1043127.00 (  7.21%)
Hmean    sembench-sem-420    981909.00 (  0.00%)  1056747.00 (  7.62%)
Hmean    sembench-sem-451    990139.00 (  0.00%)  1051609.00 (  6.21%)
Hmean    sembench-sem-482    965735.00 (  0.00%)  1040313.00 (  7.72%)

[akpm@linux-foundation.org: coding-style fixes]
[sfr@canb.auug.org.au: merge fix for WAKE_Q to DEFINE_WAKE_Q rename]
  Link: http://lkml.kernel.org/r/20161122210410.5eca9fc2@canb.auug.org.au
Link: http://lkml.kernel.org/r/1474225896-10066-3-git-send-email-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Acked-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 86 insertions, 180 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index b33dd98fde69..1ff103299c28 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -11,6 +11,7 @@
  * (c) 2001 Red Hat Inc
  * Lockless wakeup
  * (c) 2003 Manfred Spraul <manfred@colorfullife.com>
+ * (c) 2016 Davidlohr Bueso <dave@stgolabs.net>
  * Further wakeup optimizations, documentation
  * (c) 2010 Manfred Spraul <manfred@colorfullife.com>
  *
@@ -53,15 +54,11 @@
  *   Semaphores are actively given to waiting tasks (necessary for FIFO).
  *   (see update_queue())
  * - To improve the scalability, the actual wake-up calls are performed after
- *   dropping all locks. (see wake_up_sem_queue_prepare(),
- *   wake_up_sem_queue_do())
+ *   dropping all locks. (see wake_up_sem_queue_prepare())
  * - All work is done by the waker, the woken up task does not have to do
  *   anything - not even acquiring a lock or dropping a refcount.
  * - A woken up task may not even touch the semaphore array anymore, it may
  *   have been destroyed already by a semctl(RMID).
- * - The synchronizations between wake-ups due to a timeout/signal and a
- *   wake-up due to a completed semaphore operation is achieved by using an
- *   intermediate state (IN_WAKEUP).
  * - UNDO values are stored in an array (one per process and per
  *   semaphore array, lazily allocated). For backwards compatibility, multiple
  *   modes for the UNDO variables are supported (per process, per thread)
@@ -471,40 +468,6 @@ static inline void sem_rmid(struct ipc_namespace *ns, struct sem_array *s)
         ipc_rmid(&sem_ids(ns), &s->sem_perm);
 }
 
-/*
- * Lockless wakeup algorithm:
- * Without the check/retry algorithm a lockless wakeup is possible:
- * - queue.status is initialized to -EINTR before blocking.
- * - wakeup is performed by
- *      * unlinking the queue entry from the pending list
- *      * setting queue.status to IN_WAKEUP
- *        This is the notification for the blocked thread that a
- *        result value is imminent.
- *      * call wake_up_process
- *      * set queue.status to the final value.
- * - the previously blocked thread checks queue.status:
- *      * if it's IN_WAKEUP, then it must wait until the value changes
- *      * if it's not -EINTR, then the operation was completed by
- *        update_queue. semtimedop can return queue.status without
- *        performing any operation on the sem array.
- *      * otherwise it must acquire the spinlock and check what's up.
- *
- * The two-stage algorithm is necessary to protect against the following
- * races:
- * - if queue.status is set after wake_up_process, then the woken up idle
- *   thread could race forward and try (and fail) to acquire sma->lock
- *   before update_queue had a chance to set queue.status
- * - if queue.status is written before wake_up_process and if the
- *   blocked process is woken up by a signal between writing
- *   queue.status and the wake_up_process, then the woken up
- *   process could return from semtimedop and die by calling
- *   sys_exit before wake_up_process is called. Then wake_up_process
- *   will oops, because the task structure is already invalid.
- *   (yes, this happened on s390 with sysv msg).
- *
- */
-#define IN_WAKEUP       1
-
 /**
  * newary - Create a new semaphore set
  * @ns: namespace
@@ -703,51 +666,18 @@ undo:
         return result;
 }
 
-/** wake_up_sem_queue_prepare(q, error): Prepare wake-up
- * @q: queue entry that must be signaled
- * @error: Error value for the signal
- *
- * Prepare the wake-up of the queue entry q.
- */
-static void wake_up_sem_queue_prepare(struct list_head *pt,
-                                struct sem_queue *q, int error)
-{
-        if (list_empty(pt)) {
-                /*
-                 * Hold preempt off so that we don't get preempted and have the
-                 * wakee busy-wait until we're scheduled back on.
-                 */
-                preempt_disable();
-        }
-        q->status = IN_WAKEUP;
-        q->pid = error;
-
-        list_add_tail(&q->list, pt);
-}
-
-/**
- * wake_up_sem_queue_do - do the actual wake-up
- * @pt: list of tasks to be woken up
- *
- * Do the actual wake-up.
- * The function is called without any locks held, thus the semaphore array
- * could be destroyed already and the tasks can disappear as soon as the
- * status is set to the actual return code.
- */
-static void wake_up_sem_queue_do(struct list_head *pt)
+static inline void wake_up_sem_queue_prepare(struct sem_queue *q, int error,
+                                             struct wake_q_head *wake_q)
 {
-        struct sem_queue *q, *t;
-        int did_something;
-
-        did_something = !list_empty(pt);
-        list_for_each_entry_safe(q, t, pt, list) {
-                wake_up_process(q->sleeper);
-                /* q can disappear immediately after writing q->status. */
-                smp_wmb();
-                q->status = q->pid;
-        }
-        if (did_something)
-                preempt_enable();
+        wake_q_add(wake_q, q->sleeper);
+        /*
+         * Rely on the above implicit barrier, such that we can
+         * ensure that we hold reference to the task before setting
+         * q->status. Otherwise we could race with do_exit if the
+         * task is awoken by an external event before calling
+         * wake_up_process().
+         */
+        WRITE_ONCE(q->status, error);
 }
 
 static void unlink_queue(struct sem_array *sma, struct sem_queue *q)
@@ -795,18 +725,18 @@ static int check_restart(struct sem_array *sma, struct sem_queue *q)
  * wake_const_ops - wake up non-alter tasks
  * @sma: semaphore array.
  * @semnum: semaphore that was modified.
- * @pt: list head for the tasks that must be woken up.
+ * @wake_q: lockless wake-queue head.
  *
  * wake_const_ops must be called after a semaphore in a semaphore array
  * was set to 0. If complex const operations are pending, wake_const_ops must
  * be called with semnum = -1, as well as with the number of each modified
  * semaphore.
- * The tasks that must be woken up are added to @pt. The return code
+ * The tasks that must be woken up are added to @wake_q. The return code
  * is stored in q->pid.
  * The function returns 1 if at least one operation was completed successfully.
  */
 static int wake_const_ops(struct sem_array *sma, int semnum,
-                                struct list_head *pt)
+                          struct wake_q_head *wake_q)
 {
         struct sem_queue *q;
         struct list_head *walk;
@@ -832,7 +762,7 @@ static int wake_const_ops(struct sem_array *sma, int semnum,
 
                         unlink_queue(sma, q);
 
-                        wake_up_sem_queue_prepare(pt, q, error);
+                        wake_up_sem_queue_prepare(q, error, wake_q);
                         if (error == 0)
                                 semop_completed = 1;
                 }
@@ -845,14 +775,14 @@ static int wake_const_ops(struct sem_array *sma, int semnum,
  * @sma: semaphore array
  * @sops: operations that were performed
  * @nsops: number of operations
- * @pt: list head of the tasks that must be woken up.
+ * @wake_q: lockless wake-queue head
  *
  * Checks all required queue for wait-for-zero operations, based
  * on the actual changes that were performed on the semaphore array.
  * The function returns 1 if at least one operation was completed successfully.
  */
 static int do_smart_wakeup_zero(struct sem_array *sma, struct sembuf *sops,
-                                        int nsops, struct list_head *pt)
+                                int nsops, struct wake_q_head *wake_q)
 {
         int i;
         int semop_completed = 0;
@@ -865,7 +795,7 @@ static int do_smart_wakeup_zero(struct sem_array *sma, struct sembuf *sops,
 
                         if (sma->sem_base[num].semval == 0) {
                                 got_zero = 1;
-                                semop_completed |= wake_const_ops(sma, num, pt);
+                                semop_completed |= wake_const_ops(sma, num, wake_q);
                         }
                 }
         } else {
@@ -876,7 +806,7 @@ static int do_smart_wakeup_zero(struct sem_array *sma, struct sembuf *sops,
                 for (i = 0; i < sma->sem_nsems; i++) {
                         if (sma->sem_base[i].semval == 0) {
                                 got_zero = 1;
-                                semop_completed |= wake_const_ops(sma, i, pt);
+                                semop_completed |= wake_const_ops(sma, i, wake_q);
                         }
                 }
         }
@@ -885,7 +815,7 @@ static int do_smart_wakeup_zero(struct sem_array *sma, struct sembuf *sops,
          * then check the global queue, too.
          */
         if (got_zero)
-                semop_completed |= wake_const_ops(sma, -1, pt);
+                semop_completed |= wake_const_ops(sma, -1, wake_q);
 
         return semop_completed;
 }
@@ -895,19 +825,19 @@ static int do_smart_wakeup_zero(struct sem_array *sma, struct sembuf *sops,
  * update_queue - look for tasks that can be completed.
  * @sma: semaphore array.
  * @semnum: semaphore that was modified.
- * @pt: list head for the tasks that must be woken up.
+ * @wake_q: lockless wake-queue head.
  *
  * update_queue must be called after a semaphore in a semaphore array
  * was modified. If multiple semaphores were modified, update_queue must
  * be called with semnum = -1, as well as with the number of each modified
  * semaphore.
- * The tasks that must be woken up are added to @pt. The return code
+ * The tasks that must be woken up are added to @wake_q. The return code
  * is stored in q->pid.
  * The function internally checks if const operations can now succeed.
  *
  * The function return 1 if at least one semop was completed successfully.
  */
-static int update_queue(struct sem_array *sma, int semnum, struct list_head *pt)
+static int update_queue(struct sem_array *sma, int semnum, struct wake_q_head *wake_q)
 {
         struct sem_queue *q;
         struct list_head *walk;
@@ -949,11 +879,11 @@ again:
                         restart = 0;
                 } else {
                         semop_completed = 1;
-                        do_smart_wakeup_zero(sma, q->sops, q->nsops, pt);
+                        do_smart_wakeup_zero(sma, q->sops, q->nsops, wake_q);
                         restart = check_restart(sma, q);
                 }
 
-                wake_up_sem_queue_prepare(pt, q, error);
+                wake_up_sem_queue_prepare(q, error, wake_q);
                 if (restart)
                         goto again;
         }
@@ -984,24 +914,24 @@ static void set_semotime(struct sem_array *sma, struct sembuf *sops)
  * @sops: operations that were performed
  * @nsops: number of operations
  * @otime: force setting otime
- * @pt: list head of the tasks that must be woken up.
+ * @wake_q: lockless wake-queue head
  *
  * do_smart_update() does the required calls to update_queue and wakeup_zero,
  * based on the actual changes that were performed on the semaphore array.
  * Note that the function does not do the actual wake-up: the caller is
- * responsible for calling wake_up_sem_queue_do(@pt).
+ * responsible for calling wake_up_q().
  * It is safe to perform this call after dropping all locks.
  */
 static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsops,
-                        int otime, struct list_head *pt)
+                            int otime, struct wake_q_head *wake_q)
 {
         int i;
 
-        otime |= do_smart_wakeup_zero(sma, sops, nsops, pt);
+        otime |= do_smart_wakeup_zero(sma, sops, nsops, wake_q);
 
         if (!list_empty(&sma->pending_alter)) {
                 /* semaphore array uses the global queue - just process it. */
-                otime |= update_queue(sma, -1, pt);
+                otime |= update_queue(sma, -1, wake_q);
         } else {
                 if (!sops) {
                         /*
@@ -1009,7 +939,7 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
                          * known. Check all.
                          */
                         for (i = 0; i < sma->sem_nsems; i++)
-                                otime |= update_queue(sma, i, pt);
+                                otime |= update_queue(sma, i, wake_q);
                 } else {
                         /*
                          * Check the semaphores that were increased:
@@ -1023,7 +953,7 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
                         for (i = 0; i < nsops; i++) {
                                 if (sops[i].sem_op > 0) {
                                         otime |= update_queue(sma,
-                                                        sops[i].sem_num, pt);
+                                                              sops[i].sem_num, wake_q);
                                 }
                         }
                 }
@@ -1111,8 +1041,8 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
         struct sem_undo *un, *tu;
         struct sem_queue *q, *tq;
         struct sem_array *sma = container_of(ipcp, struct sem_array, sem_perm);
-        struct list_head tasks;
         int i;
+        DEFINE_WAKE_Q(wake_q);
 
         /* Free the existing undo structures for this semaphore set.  */
         ipc_assert_locked_object(&sma->sem_perm);
@@ -1126,25 +1056,24 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
         }
 
         /* Wake up all pending processes and let them fail with EIDRM. */
-        INIT_LIST_HEAD(&tasks);
         list_for_each_entry_safe(q, tq, &sma->pending_const, list) {
                 unlink_queue(sma, q);
-                wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
+                wake_up_sem_queue_prepare(q, -EIDRM, &wake_q);
         }
 
         list_for_each_entry_safe(q, tq, &sma->pending_alter, list) {
                 unlink_queue(sma, q);
-                wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
+                wake_up_sem_queue_prepare(q, -EIDRM, &wake_q);
         }
         for (i = 0; i < sma->sem_nsems; i++) {
                 struct sem *sem = sma->sem_base + i;
                 list_for_each_entry_safe(q, tq, &sem->pending_const, list) {
                         unlink_queue(sma, q);
-                        wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
+                        wake_up_sem_queue_prepare(q, -EIDRM, &wake_q);
                 }
                 list_for_each_entry_safe(q, tq, &sem->pending_alter, list) {
                         unlink_queue(sma, q);
-                        wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
+                        wake_up_sem_queue_prepare(q, -EIDRM, &wake_q);
                 }
         }
 
@@ -1153,7 +1082,7 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
         sem_unlock(sma, -1);
         rcu_read_unlock();
 
-        wake_up_sem_queue_do(&tasks);
+        wake_up_q(&wake_q);
         ns->used_sems -= sma->sem_nsems;
         ipc_rcu_putref(sma, sem_rcu_free);
 }
@@ -1292,9 +1221,9 @@ static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
         struct sem_undo *un;
         struct sem_array *sma;
         struct sem *curr;
-        int err;
-        struct list_head tasks;
-        int val;
+        int err, val;
+        DEFINE_WAKE_Q(wake_q);
+
 #if defined(CONFIG_64BIT) && defined(__BIG_ENDIAN)
         /* big-endian 64bit */
         val = arg >> 32;
@@ -1306,8 +1235,6 @@ static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
         if (val > SEMVMX || val < 0)
                 return -ERANGE;
 
-        INIT_LIST_HEAD(&tasks);
-
         rcu_read_lock();
         sma = sem_obtain_object_check(ns, semid);
         if (IS_ERR(sma)) {
@@ -1350,10 +1277,10 @@ static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
         curr->sempid = task_tgid_vnr(current);
         sma->sem_ctime = get_seconds();
         /* maybe some queued-up processes were waiting for this */
-        do_smart_update(sma, NULL, 0, 0, &tasks);
+        do_smart_update(sma, NULL, 0, 0, &wake_q);
         sem_unlock(sma, -1);
         rcu_read_unlock();
-        wake_up_sem_queue_do(&tasks);
+        wake_up_q(&wake_q);
         return 0;
 }
 
@@ -1365,9 +1292,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
         int err, nsems;
         ushort fast_sem_io[SEMMSL_FAST];
         ushort *sem_io = fast_sem_io;
-        struct list_head tasks;
-
-        INIT_LIST_HEAD(&tasks);
+        DEFINE_WAKE_Q(wake_q);
 
         rcu_read_lock();
         sma = sem_obtain_object_check(ns, semid);
@@ -1478,7 +1403,7 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
                 }
                 sma->sem_ctime = get_seconds();
                 /* maybe some queued-up processes were waiting for this */
-                do_smart_update(sma, NULL, 0, 0, &tasks);
+                do_smart_update(sma, NULL, 0, 0, &wake_q);
                 err = 0;
                 goto out_unlock;
         }
@@ -1514,7 +1439,7 @@ out_unlock:
         sem_unlock(sma, -1);
 out_rcu_wakeup:
         rcu_read_unlock();
-        wake_up_sem_queue_do(&tasks);
+        wake_up_q(&wake_q);
 out_free:
         if (sem_io != fast_sem_io)
                 ipc_free(sem_io);
@@ -1787,32 +1712,6 @@ out:
         return un;
 }
 
-
-/**
- * get_queue_result - retrieve the result code from sem_queue
- * @q: Pointer to queue structure
- *
- * Retrieve the return code from the pending queue. If IN_WAKEUP is found in
- * q->status, then we must loop until the value is replaced with the final
- * value: This may happen if a task is woken up by an unrelated event (e.g.
- * signal) and in parallel the task is woken up by another task because it got
- * the requested semaphores.
- *
- * The function can be called with or without holding the semaphore spinlock.
- */
-static int get_queue_result(struct sem_queue *q)
-{
-        int error;
-
-        error = q->status;
-        while (unlikely(error == IN_WAKEUP)) {
-                cpu_relax();
-                error = q->status;
-        }
-
-        return error;
-}
-
 SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
                 unsigned, nsops, const struct timespec __user *, timeout)
 {
@@ -1825,7 +1724,6 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
         struct sem_queue queue;
         unsigned long jiffies_left = 0;
         struct ipc_namespace *ns;
-        struct list_head tasks;
 
         ns = current->nsproxy->ipc_ns;
 
@@ -1865,7 +1763,6 @@ 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 */
@@ -1933,22 +1830,31 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
         queue.alter = alter;
 
         error = perform_atomic_semop(sma, &queue);
-        if (error == 0) {
-                /* If the operation was successful, then do
+        if (error == 0) { /* non-blocking succesfull path */
+                DEFINE_WAKE_Q(wake_q);
+
+                /*
+                 * If the operation was successful, then do
                  * the required updates.
                  */
                 if (alter)
-                        do_smart_update(sma, sops, nsops, 1, &tasks);
+                        do_smart_update(sma, sops, nsops, 1, &wake_q);
                 else
                         set_semotime(sma, sops);
+
+                sem_unlock(sma, locknum);
+                rcu_read_unlock();
+                wake_up_q(&wake_q);
+
+                goto out_free;
         }
-        if (error <= 0)
+        if (error < 0) /* non-blocking error path */
                 goto out_unlock_free;
 
-        /* We need to sleep on this operation, so we put the current
+        /*
+         * We need to sleep on this operation, so we put the current
          * task into the pending queue and go to sleep.
          */
-
         if (nsops == 1) {
                 struct sem *curr;
                 curr = &sma->sem_base[sops->sem_num];
@@ -1977,10 +1883,10 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
                 sma->complex_count++;
         }
 
+sleep_again:
         queue.status = -EINTR;
         queue.sleeper = current;
 
-sleep_again:
         __set_current_state(TASK_INTERRUPTIBLE);
         sem_unlock(sma, locknum);
         rcu_read_unlock();
@@ -1990,28 +1896,31 @@ sleep_again:
         else
                 schedule();
 
-        error = get_queue_result(&queue);
-
+        /*
+         * fastpath: the semop has completed, either successfully or not, from
+         * the syscall pov, is quite irrelevant to us at this point; we're done.
+         *
+         * We _do_ care, nonetheless, about being awoken by a signal or
+         * spuriously.  The queue.status is checked again in the slowpath (aka
+         * after taking sem_lock), such that we can detect scenarios where we
+         * were awakened externally, during the window between wake_q_add() and
+         * wake_up_q().
+         */
+        error = READ_ONCE(queue.status);
         if (error != -EINTR) {
-                /* fast path: update_queue already obtained all requested
-                 * resources.
-                 * Perform a smp_mb(): User space could assume that semop()
-                 * is a memory barrier: Without the mb(), the cpu could
-                 * speculatively read in user space stale data that was
-                 * overwritten by the previous owner of the semaphore.
+                /*
+                 * User space could assume that semop() is a memory barrier:
+                 * Without the mb(), the cpu could speculatively read in user
+                 * space stale data that was overwritten by the previous owner
+                 * of the semaphore.
                  */
                 smp_mb();
-
                 goto out_free;
         }
 
         rcu_read_lock();
         sma = sem_obtain_lock(ns, semid, sops, nsops, &locknum);
-
-        /*
-         * Wait until it's guaranteed that no wakeup_sem_queue_do() is ongoing.
-         */
-        error = get_queue_result(&queue);
+        error = READ_ONCE(queue.status);
 
         /*
          * Array removed? If yes, leave without sem_unlock().
@@ -2021,7 +1930,6 @@ sleep_again:
                 goto out_free;
         }
 
-
         /*
          * If queue.status != -EINTR we are woken up by another process.
          * Leave without unlink_queue(), but with sem_unlock().
@@ -2030,13 +1938,13 @@ sleep_again:
                 goto out_unlock_free;
 
         /*
-         * If an interrupt occurred we have to clean up the queue
+         * If an interrupt occurred we have to clean up the queue.
          */
         if (timeout && jiffies_left == 0)
                 error = -EAGAIN;
 
         /*
-         * If the wakeup was spurious, just retry
+         * If the wakeup was spurious, just retry.
          */
         if (error == -EINTR && !signal_pending(current))
                 goto sleep_again;
@@ -2046,7 +1954,6 @@ sleep_again:
 out_unlock_free:
         sem_unlock(sma, locknum);
         rcu_read_unlock();
-        wake_up_sem_queue_do(&tasks);
 out_free:
         if (sops != fast_sops)
                 kfree(sops);
@@ -2107,8 +2014,8 @@ void exit_sem(struct task_struct *tsk)
         for (;;) {
                 struct sem_array *sma;
                 struct sem_undo *un;
-                struct list_head tasks;
                 int semid, i;
+                DEFINE_WAKE_Q(wake_q);
 
                 cond_resched();
 
@@ -2196,11 +2103,10 @@ void exit_sem(struct task_struct *tsk)
                         }
                 }
                 /* maybe some queued-up processes were waiting for this */
-                INIT_LIST_HEAD(&tasks);
-                do_smart_update(sma, NULL, 0, 1, &tasks);
+                do_smart_update(sma, NULL, 0, 1, &wake_q);
                 sem_unlock(sma, -1);
                 rcu_read_unlock();
-                wake_up_sem_queue_do(&tasks);
+                wake_up_q(&wake_q);
 
                 kfree_rcu(un, rcu);
         }