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ipc/sem: rework task wakeups

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Our sysv sems have been using the notion of lockless wakeups for a
while, ever since commit 0a2b9d4c79 ("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>
This commit is contained in:
Davidlohr Bueso 2016-12-14 15:06:34 -08:00 committed by Linus Torvalds
parent 248e7357cf
commit 9ae949fa38
1 changed files with 86 additions and 180 deletions
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266
ipc/sem.c
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@ -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)
static inline void wake_up_sem_queue_prepare(struct sem_queue *q, int error,
struct wake_q_head *wake_q)
{
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)
{
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);
}