On Tue, Apr 29, 2014 at 07:44:06PM +1000, NeilBrown wrote:
>
>
> It is currently not possible for various wait_on_bit functions to
> implement a timeout.
> While the "action" function that is called to do the waiting could
> certainly use schedule_timeout(), there is no way to carry forward the
> remaining timeout after a false wake-up.
> As false-wakeups a clearly possible at least due to possible
> hash collisions in bit_waitqueue(), this is a real problem.
>
> The 'action' function is currently passed a pointer to the word
> containing the bit being waited on. Of the 27 currently defined
> action functions, zero of them use this pointer.
> So changing it to something else will be a little noisy but will have
> no immediate effect.
>
> This patch changes the 'action' function to take a pointer to the
> "struct wait_bit_key", which contains a pointer to the word
> containing the bit so nothing is really lost.
>
> It also adds a 'private' field to "struct wait_bit_key", which is
> initialized to zero.
>
> An action function can now implement a timeout with something like
>
> static int timed_out_waiter(struct wait_bit_key *key)
> {
> unsigned long waited;
> if (key->private == 0) {
> key->private = jiffies;
> if (key->private == 0)
> key->private -= 1;
> }
> waited = jiffies - key->private;
> if (waited > 10 * HZ)
> return -EAGAIN;
> schedule_timeout(waited - 10 * HZ);
> return 0;
> }
>
> If any other need for context in a waiter were found it would be easy
> to use ->private for some other purpose, or even extend "struct
> wait_bit_key".
>
> My particular need is to support timeouts in nfs_release_page() to
> avoid deadlocks with loopback mounted NFS.
So I'm sure I'm not getting it; but why is all the wait_bit crap so
entirely different from the normal wait stuff?
Surely something like:
wait_event_timeout(&wq, test_bit(bit, word), timeout);
Is pretty much the same, no? The only thing that's different is the wake
function, but surely we can thread that into there somehow without all
this silly repetition.
Furthermore, I count about 23 action functions, of which there appear to
be only like 4 actual variants. Surely such repetition sucks arse and
should be avoided?
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