Messages in this thread | | | Date | Mon, 21 Jan 2019 08:02:18 -0800 | From | Davidlohr Bueso <> | Subject | Re: [PATCH v4] sched/wake_q: Reduce reference counting for special users |
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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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