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    SubjectRe: [PATCH v4] sched/wake_q: Reduce reference counting for special users
    Hi - considering that the wake_q patches were picked up for tip/urgent, can
    this one make it in as well?

    Thanks,
    Davidlohr

    On Tue, 18 Dec 2018, Waiman Long wrote:

    >On 12/18/2018 02:53 PM, Davidlohr Bueso wrote:
    >> Some users, specifically futexes and rwsems, required fixes
    >> that allowed the callers to be safe when wakeups occur before
    >> they are expected by wake_up_q(). Such scenarios also play
    >> games and rely on reference counting, and until now were
    >> pivoting on wake_q doing it. With the wake_q_add() call being
    >> moved down, this can no longer be the case. As such we end up
    >> with a a double task refcounting overhead; and these callers
    >> care enough about this (being rather core-ish).
    >>
    >> This patch introduces a wake_q_add_safe() call that serves
    >> for callers that have already done refcounting and therefore the
    >> task is 'safe' from wake_q point of view (int that it requires
    >> reference throughout the entire queue/>wakeup cycle). In the one
    >> case it has internal reference counting, in the other case it
    >> consumes the reference counting.
    >>
    >> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
    >> ---
    >>
    >> - Changes from v3: fixed wake_q_add_safe. While previous version
    >>  had been tested with a bootup, the failed cmpxchg path obviously
    >>  hadn't been exercised.  Sorry about the noise.
    >>
    >> include/linux/sched/wake_q.h |  4 +--
    >> kernel/futex.c               |  3 +--
    >> kernel/locking/rwsem-xadd.c  |  4 +--
    >> kernel/sched/core.c          | 60
    >> ++++++++++++++++++++++++++++++++------------
    >> 4 files changed, 48 insertions(+), 23 deletions(-)
    >>
    >> diff --git a/include/linux/sched/wake_q.h b/include/linux/sched/wake_q.h
    >> index 545f37138057..ad826d2a4557 100644
    >> --- a/include/linux/sched/wake_q.h
    >> +++ b/include/linux/sched/wake_q.h
    >> @@ -51,8 +51,8 @@ static inline void wake_q_init(struct wake_q_head
    >> *head)
    >>     head->lastp = &head->first;
    >> }
    >>
    >> -extern void wake_q_add(struct wake_q_head *head,
    >> -               struct task_struct *task);
    >> +extern void wake_q_add(struct wake_q_head *head, struct task_struct
    >> *task);
    >> +extern void wake_q_add_safe(struct wake_q_head *head, struct
    >> task_struct *task);
    >> extern void wake_up_q(struct wake_q_head *head);
    >>
    >> #endif /* _LINUX_SCHED_WAKE_Q_H */
    >> diff --git a/kernel/futex.c b/kernel/futex.c
    >> index d14971f6ed3d..6218d98f649b 100644
    >> --- a/kernel/futex.c
    >> +++ b/kernel/futex.c
    >> @@ -1402,8 +1402,7 @@ static void mark_wake_futex(struct wake_q_head
    >> *wake_q, struct futex_q *q)
    >>      * Queue the task for later wakeup for after we've released
    >>      * the hb->lock. wake_q_add() grabs reference to p.
    >>      */
    >> -    wake_q_add(wake_q, p);
    >> -    put_task_struct(p);
    >> +    wake_q_add_safe(wake_q, p);
    >> }
    >>
    >> /*
    >> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
    >> index 50d9af615dc4..fbe96341beee 100644
    >> --- a/kernel/locking/rwsem-xadd.c
    >> +++ b/kernel/locking/rwsem-xadd.c
    >> @@ -211,9 +211,7 @@ static void __rwsem_mark_wake(struct rw_semaphore
    >> *sem,
    >>          * Ensure issuing the wakeup (either by us or someone else)
    >>          * after setting the reader waiter to nil.
    >>          */
    >> -        wake_q_add(wake_q, tsk);
    >> -        /* wake_q_add() already take the task ref */
    >> -        put_task_struct(tsk);
    >> +        wake_q_add_safe(wake_q, tsk);
    >>     }
    >>
    >>     adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
    >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    >> index d740d7a3608d..be977df66a21 100644
    >> --- a/kernel/sched/core.c
    >> +++ b/kernel/sched/core.c
    >> @@ -396,19 +396,7 @@ static bool set_nr_if_polling(struct task_struct *p)
    >> #endif
    >> #endif
    >>
    >> -/**
    >> - * wake_q_add() - queue a wakeup for 'later' waking.
    >> - * @head: the wake_q_head to add @task to
    >> - * @task: the task to queue for 'later' wakeup
    >> - *
    >> - * Queue a task for later wakeup, most likely by the wake_up_q() call
    >> in the
    >> - * same context, _HOWEVER_ this is not guaranteed, the wakeup can come
    >> - * instantly.
    >> - *
    >> - * This function must be used as-if it were wake_up_process(); IOW
    >> the task
    >> - * must be ready to be woken at this location.
    >> - */
    >> -void wake_q_add(struct wake_q_head *head, struct task_struct *task)
    >> +static bool __wake_q_add(struct wake_q_head *head, struct task_struct
    >> *task)
    >> {
    >>     struct wake_q_node *node = &task->wake_q;
    >>
    >> @@ -422,15 +410,55 @@ void wake_q_add(struct wake_q_head *head, struct
    >> task_struct *task)
    >>      */
    >>     smp_mb__before_atomic();
    >>     if (unlikely(cmpxchg_relaxed(&node->next, NULL, WAKE_Q_TAIL)))
    >> -        return;
    >> -
    >> -    get_task_struct(task);
    >> +        return false;
    >>
    >>     /*
    >>      * The head is context local, there can be no concurrency.
    >>      */
    >>     *head->lastp = node;
    >>     head->lastp = &node->next;
    >> +    return true;
    >> +}
    >> +
    >> +/**
    >> + * wake_q_add() - queue a wakeup for 'later' waking.
    >> + * @head: the wake_q_head to add @task to
    >> + * @task: the task to queue for 'later' wakeup
    >> + *
    >> + * Queue a task for later wakeup, most likely by the wake_up_q() call
    >> in the
    >> + * same context, _HOWEVER_ this is not guaranteed, the wakeup can come
    >> + * instantly.
    >> + *
    >> + * This function must be used as-if it were wake_up_process(); IOW
    >> the task
    >> + * must be ready to be woken at this location.
    >> + */
    >> +void wake_q_add(struct wake_q_head *head, struct task_struct *task)
    >> +{
    >> +    if (__wake_q_add(head, task))
    >> +        get_task_struct(task);
    >> +}
    >> +
    >> +/**
    >> + * wake_q_add_safe() - safely queue a wakeup for 'later' waking.
    >> + * @head: the wake_q_head to add @task to
    >> + * @task: the task to queue for 'later' wakeup
    >> + *
    >> + * Queue a task for later wakeup, most likely by the wake_up_q() call
    >> in the
    >> + * same context, _HOWEVER_ this is not guaranteed, the wakeup can come
    >> + * instantly.
    >> + *
    >> + * This function must be used as-if it were wake_up_process(); IOW
    >> the task
    >> + * must be ready to be woken at this location.
    >> + *
    >> + * This function is essentially a task-safe equivalent to
    >> wake_q_add(). Callers
    >> + * that already hold reference to @task can call the 'safe' version
    >> and trust
    >> + * wake_q to do the right thing depending whether or not the @task is
    >> already
    >> + * queued for wakeup.
    >> + */
    >> +void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task)
    >> +{
    >> +    if (!__wake_q_add(head, task))
    >> +        put_task_struct(task);
    >> }
    >>
    >> void wake_up_q(struct wake_q_head *head)
    >
    >Acked-by: Waiman Long <longman@redhat.com>
    >

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