Re: [PATCH-tip v6 01/20] locking/rwsem: Prevent decrement of reader count before increment

[Waiman Long]

On 4/28/19 1:59 PM, Linus Torvalds wrote:
> On Sun, Apr 28, 2019 at 10:41 AM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> It's the *first* loop that you could play games with, because you hold
>> the lock, and the list is stable during that loop. So the *first* loop
>> could just walk the list, and then do one list splitting operation
>> instead of doing that "list_move_tail()" thing for each entry.
> .. having looked at that, I would suggest against it.
>
> I _think_ this short and sweet code snippet might just work fine for
> the first loop:
>
>         list_for_each_entry(waiter, &sem->wait_list, list) {
>                 if (waiter->type == RWSEM_WAITING_FOR_WRITE)
>                         break;
>                 woken++;
>         }
>         list_cut_before(&wlist, &sem->wait_list, waiter);
>
> and if it *does* work it would be both smaller and more efficient. But
> it looks a bit too subtle to my taste. Somebody would need to go
> through that with a fine comb, and double-check that it gets the
> "whole list" case right, for example.
>
> So the "phase 1" loop could be perhaps simplified to the above cute things.
>
> But the "phase 2" loop absolutely has to be changed to use
> list_for_each_entry_safe().
>
>                Linus

You are right. Thanks for checking my code. I will make the necessary
change.

Cheers,
Longman