Messages in this thread | | | Date | Sun, 26 Feb 2023 13:00:26 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 3/6] locking/rwsem: Rework writer wakeup |
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On Thu, Feb 23, 2023 at 04:38:08PM -0500, Waiman Long wrote:
> > @@ -1143,54 +1138,36 @@ rwsem_down_write_slowpath(struct rw_sema > > } else { > > atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count); > > } > > + raw_spin_unlock_irq(&sem->wait_lock); > > /* wait until we successfully acquire the lock */ > > - set_current_state(state); > > trace_contention_begin(sem, LCB_F_WRITE); > > for (;;) { > > - if (rwsem_try_write_lock(sem, &waiter)) { > > - /* rwsem_try_write_lock() implies ACQUIRE on success */ > > + set_current_state(state); > > + if (!smp_load_acquire(&waiter.task)) { > > + /* Matches rwsem_waiter_wake()'s smp_store_release(). */ > > break; > > } > > - > > - raw_spin_unlock_irq(&sem->wait_lock); > > - > > - if (signal_pending_state(state, current)) > > - goto out_nolock; > > - > > - /* > > - * After setting the handoff bit and failing to acquire > > - * the lock, attempt to spin on owner to accelerate lock > > - * transfer. If the previous owner is a on-cpu writer and it > > - * has just released the lock, OWNER_NULL will be returned. > > - * In this case, we attempt to acquire the lock again > > - * without sleeping. > > - */ > > - if (waiter.handoff_set) { > > - enum owner_state owner_state; > > - > > - owner_state = rwsem_spin_on_owner(sem); > > - if (owner_state == OWNER_NULL) > > - goto trylock_again; > > + if (signal_pending_state(state, current)) { > > + raw_spin_lock_irq(&sem->wait_lock); > > + if (waiter.task) > > + goto out_nolock; > > + raw_spin_unlock_irq(&sem->wait_lock); > > + /* Ordered by sem->wait_lock against rwsem_mark_wake(). */ > > + break; > > } > > - > > schedule_preempt_disabled(); > > lockevent_inc(rwsem_sleep_writer); > > - set_current_state(state); > > -trylock_again: > > - raw_spin_lock_irq(&sem->wait_lock); > > } > > __set_current_state(TASK_RUNNING); > > - raw_spin_unlock_irq(&sem->wait_lock); > > lockevent_inc(rwsem_wlock); > > trace_contention_end(sem, 0); > > return sem; > > out_nolock: > > - __set_current_state(TASK_RUNNING); > > - raw_spin_lock_irq(&sem->wait_lock); > > rwsem_del_wake_waiter(sem, &waiter, &wake_q); > > + __set_current_state(TASK_RUNNING); > > lockevent_inc(rwsem_wlock_fail); > > trace_contention_end(sem, -EINTR); > > return ERR_PTR(-EINTR); > > I believe it is better to change state inside the wait_lock critical section > to provide a release barrier for free.
I can't follow... a release for what? Note that the reader slowpath has this exact form already.
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