On 04/17/2019 04:05 AM, Peter Zijlstra wrote: > On Tue, Apr 16, 2019 at 02:16:11PM -0400, Waiman Long wrote: > >>>> @@ -608,56 +687,63 @@ __rwsem_down_write_failed_common(struct rw_semaphore >>>> *sem, int state) >>>> */ >>>> waiter.task = current; >>>> waiter.type = RWSEM_WAITING_FOR_WRITE; >>>> + waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT; >>>> >>>> raw_spin_lock_irq(&sem->wait_lock); >>>> >>>> /* account for this before adding a new element to the list */ >>>> + wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST; >>>> >>>> list_add_tail(&waiter.list, &sem->wait_list); >>>> >>>> /* we're now waiting on the lock */ >>>> + if (wstate == WRITER_NOT_FIRST) { >>>> count = atomic_long_read(&sem->count); >>>> >>>> /* >>>> + * If there were already threads queued before us and: >>>> + * 1) there are no no active locks, wake the front >>>> + * queued process(es) as the handoff bit might be set. >>>> + * 2) there are no active writers and some readers, the lock >>>> + * must be read owned; so we try to wake any read lock >>>> + * waiters that were queued ahead of us. >>>> */ >>>> + if (!RWSEM_COUNT_LOCKED(count)) >>>> + __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); >>>> + else if (!(count & RWSEM_WRITER_MASK) && >>>> + (count & RWSEM_READER_MASK)) >>>> __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q); >>> Does the above want to be something like: >>> >>> if (!(count & RWSEM_WRITER_LOCKED)) { >>> __rwsem_mark_wake(sem, (count & RWSEM_READER_MASK) ? >>> RWSEM_WAKE_READERS : >>> RWSEM_WAKE_ANY, &wake_q); >>> } >> Yes. >> >>>> + else >>>> + goto wait; >>>> >>>> + /* >>>> + * The wakeup is normally called _after_ the wait_lock >>>> + * is released, but given that we are proactively waking >>>> + * readers we can deal with the wake_q overhead as it is >>>> + * similar to releasing and taking the wait_lock again >>>> + * for attempting rwsem_try_write_lock(). >>>> + */ >>>> + wake_up_q(&wake_q); >>> Hurmph.. the reason we do wake_up_q() outside of wait_lock is such that >>> those tasks don't bounce on wait_lock. Also, it removes a great deal of >>> hold-time from wait_lock. >>> >>> So I'm not sure I buy your argument here. >>> >> Actually, we don't want to release the wait_lock, do wake_up_q() and >> acquire the wait_lock again as the state would have been changed. I >> didn't change the comment on this patch, but will reword it to discuss that. > I don't understand, we've queued ourselves, we're on the list, we're not > first. How would dropping the lock to try and kick waiters before us be > a problem? > > Sure, once we re-acquire the lock we have to re-avaluate @wstate to see > if we're first now or not, but we need to do that anyway. > > So what is wrong with the below? > > --- a/include/linux/sched/wake_q.h > +++ b/include/linux/sched/wake_q.h > @@ -51,6 +51,11 @@ static inline void wake_q_init(struct wa > head->lastp = &head->first; > } > > +static inline bool wake_q_empty(struct wake_q_head *head) > +{ > + return head->first == WAKE_Q_TAIL; > +} > + > 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); > --- a/kernel/locking/rwsem.c > +++ b/kernel/locking/rwsem.c > @@ -700,25 +700,22 @@ __rwsem_down_write_failed_common(struct > * must be read owned; so we try to wake any read lock > * waiters that were queued ahead of us. > */ > - if (!(count & RWSEM_LOCKED_MASK)) > - __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); > - else if (!(count & RWSEM_WRITER_MASK) && > - (count & RWSEM_READER_MASK)) > - __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q); > - else > + if (count & RWSEM_WRITER_LOCKED) > goto wait; > - /* > - * The wakeup is normally called _after_ the wait_lock > - * is released, but given that we are proactively waking > - * readers we can deal with the wake_q overhead as it is > - * similar to releasing and taking the wait_lock again > - * for attempting rwsem_try_write_lock(). > - */ > - wake_up_q(&wake_q); > - /* > - * Reinitialize wake_q after use. > - */ > - wake_q_init(&wake_q); > + > + __rwsem_mark_wake(sem, (count & RWSEM_READER_MASK) ? > + RWSEM_WAKE_READERS : > + RWSEM_WAKE_ANY, &wake_q); > + > + if (!wake_q_empty(&wake_q)) { > + raw_spin_unlock_irq(&sem->wait_lock); > + wake_up_q(&wake_q); > + /* used again, reinit */ > + wake_q_init(&wake_q); > + raw_spin_lock_irq(&sem->wait_lock); > + if (rwsem_waiter_is_first(sem, &waiter)) > + wstate = WRITER_FIRST; > + } > } else { > count = atomic_long_add_return(RWSEM_FLAG_WAITERS, &sem->count); > }
Yes, we can certainly do that. My point is that I haven't changed the existing logic regarding that wakeup, I only move it around in the patch. As it is not related to lock handoff, we can do it as a separate patch. Cheers, Longman