Messages in this thread | | | Date | Thu, 9 Nov 2006 11:42:49 -0800 (PST) | From | Davide Libenzi <> | Subject | Re: [take24 3/6] kevent: poll/select() notifications. |
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On Thu, 9 Nov 2006, Evgeniy Polyakov wrote:
> On Thu, Nov 09, 2006 at 10:51:56AM -0800, Davide Libenzi (davidel@xmailserver.org) wrote: > > On Thu, 9 Nov 2006, Evgeniy Polyakov wrote: > > > > > +static int kevent_poll_callback(struct kevent *k) > > > +{ > > > + if (k->event.req_flags & KEVENT_REQ_LAST_CHECK) { > > > + return 1; > > > + } else { > > > + struct file *file = k->st->origin; > > > + unsigned int revents = file->f_op->poll(file, NULL); > > > + > > > + k->event.ret_data[0] = revents & k->event.event; > > > + > > > + return (revents & k->event.event); > > > + } > > > +} > > > > You need to be careful that file->f_op->poll is not called inside the > > spin_lock_irqsave/spin_lock_irqrestore pair, since (even this came up > > during epoll developemtn days) file->f_op->poll might do a simple > > spin_lock_irq/spin_unlock_irq. This unfortunate constrain forced epoll to > > have a suboptimal double O(R) loop to handle LT events. > > It is tricky - users call wake_up() from any context, which in turn ends > up calling kevent_storage_ready(), which calls kevent_poll_callback() with > KEVENT_REQ_LAST_CHECK bit set, which becomes almost empty call in fast > path. Since callback returns 1, kevent will be queued into ready queue, > which is processed on behalf of syscalls - in that case kevent will > check the flag and since KEVENT_REQ_LAST_CHECK is set, will call > callback again to check if kevent is correctly marked, but already > without that flag (it happens in syscall context, i.e. process context > without any locks held), so callback calls ->poll(), which can sleep, > but it is safe. If ->poll() returns 'ready' value, kevent is transfers > data into userspace, otherwise it is 'requeued' (just removed from > ready queue).
Oh, mine was only a general warn. I hadn't looked at the generic code before. But now that I poke on it, I see:
void kevent_requeue(struct kevent *k) { unsigned long flags;
spin_lock_irqsave(&k->st->lock, flags); __kevent_requeue(k, 0); spin_unlock_irqrestore(&k->st->lock, flags); }
and then:
static int __kevent_requeue(struct kevent *k, u32 event) { int ret, rem; unsigned long flags;
ret = k->callbacks.callback(k);
Isn't the k->callbacks.callback() possibly end up calling f_op->poll?
- Davide
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