Messages in this thread | | | Date | Thu, 15 May 2014 17:34:24 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v4 |
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On 05/15, Peter Zijlstra wrote: > > So I suppose I'm failing to see the problem with something like:
Yeeees, I was thinking about something like this too ;)
> static inline void lock_page(struct page *page) > { > if (!trylock_page(page)) > __lock_page(page); > } > > static inline void unlock_page(struct page *page) > { > clear_bit_unlock(&page->flags, PG_locked); > if (PageWaiters(page)) > __unlock_page(); > }
but in this case we need mb() before PageWaiters(), I guess.
> void __lock_page(struct page *page) > { > struct wait_queue_head_t *wqh = page_waitqueue(page); > DEFINE_WAIT_BIT(wait, &page->flags, PG_locked); > > spin_lock_irq(&wqh->lock); > if (!PageWaiters(page)) > SetPageWaiters(page); > > wait.flags |= WQ_FLAG_EXCLUSIVE; > preempt_disable();
why?
> do { > if (list_empty(&wait->task_list)) > __add_wait_queue_tail(wqh, &wait); > > set_current_state(TASK_UNINTERRUPTIBLE); > > if (test_bit(wait.key.bit_nr, wait.key.flags)) { > spin_unlock_irq(&wqh->lock); > schedule_preempt_disabled(); > spin_lock_irq(&wqh->lock);
OK, probably to avoid the preemption before schedule(). Still can't undestand why this makes sense, but in this case it would be better to do disable/enable under "if (test_bit())" ?
Of course, this needs more work for lock_page_killable(), but this should be simple.
Oleg.
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