On Wed, Jan 10, 2018 at 09:04:16PM +0000, Al Viro wrote:
> On Wed, Jan 10, 2018 at 04:58:24PM +0100, Christoph Hellwig wrote:
> > ->get_poll_head returns the waitqueue that the poll operation is going
> > to sleep on. Note that this means we can only use a single waitqueue
> > for the poll, unlike some current drivers that use two waitqueues for
> > different events. But now that we have keyed wakeups and heavily use
> > those for poll there aren't that many good reason left to keep the
> > multiple waitqueues, and if there are any ->poll is still around, the
> > driver just won't support aio poll.
>
> There's another problem with that - currently ->poll() may tell you "sod off,
> I've got nothing for you to sleep on, eat your POLLHUP|POLLERR|something
> and don't pester me again". With your API that's hard to express sanely.
>
> Another piece of fun related to that is handling of disconnects in general -
>
> static __poll_t proc_reg_poll(struct file *file, struct poll_table_struct *pts)
> {
> struct proc_dir_entry *pde = PDE(file_inode(file));
> __poll_t rv = DEFAULT_POLLMASK;
> __poll_t (*poll)(struct file *, struct poll_table_struct *);
> if (use_pde(pde)) {
> poll = pde->proc_fops->poll;
> if (poll)
> rv = poll(file, pts);
> unuse_pde(pde);
> }
> return rv;
> }
>
> and similar in sysfs.
>
> Note, BTW, the places like wait->_qproc = NULL; in do_select() and its ilk.
> Some of them are "don't bother putting me on any queues, I won't be sleeping
> anyway". Some are "I'm already on all queues I care about, I'm going to
> sleep now and the query everything again once woken up". It would be nice
> to have the method splitup reflect that kind of logics...
>
> What about af_alg_poll(), BTW? Looks like you've missed that one...
>
> Another thing: IMO file_can_poll() should use FMODE_CAN_POLL - either as
> "true if set, otherwise check ->f_op and set accordingly" or set in
> do_dentry_open() and just check it in file_can_poll()...
Whee... The very first ->poll() instance in alphabetic order on pathnames:
in arch/cris/arch-v10/drivers/gpio.c
static __poll_t gpio_poll(struct file *file, poll_table *wait)
{
__poll_t mask = 0;
struct gpio_private *priv = file->private_data;
unsigned long data;
unsigned long flags;
spin_lock_irqsave(&gpio_lock, flags);
poll_wait(file, &priv->alarm_wq, wait);
IOW, we are doing poll_wait() (== possible GFP_KERNEL __get_free_page()) under
a spinlock...
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