Re: [RFC PATCH] epoll: Add synchronous wakeup support for ep_poll_callback

[Brian Geffon <bgeffon@google.com>]

We've also observed this issue on ChromeOS, it seems like it might long-standing epoll bug as it diverges from the behavior of poll. Any chance a maintainer can take a look?

Thanks
Brian

On Fri, Apr 26, 2024 at 04:05:48PM +0800, Xuewen Yan wrote:
> Now, the epoll only use wake_up() interface to wake up task.
> However, sometimes, there are epoll users which want to use
> the synchronous wakeup flag to hint the scheduler, such as
> Android binder driver.
> So add a wake_up_sync() define, and use the wake_up_sync()
> when the sync is true in ep_poll_callback().
> 
> Co-developed-by: Jing Xia <jing.xia@unisoc.com>
> Signed-off-by: Jing Xia <jing.xia@unisoc.com>
> Signed-off-by: Xuewen Yan <xuewen.yan@unisoc.com>
> ---
>  fs/eventpoll.c       | 5 ++++-
>  include/linux/wait.h | 1 +
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 882b89edc52a..9b815e0a1ac5 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1336,7 +1336,10 @@ static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, v
>  				break;
>  			}
>  		}
> -		wake_up(&ep->wq);
> +		if (sync)
> +			wake_up_sync(&ep->wq);
> +		else
> +			wake_up(&ep->wq);
>  	}
>  	if (waitqueue_active(&ep->poll_wait))
>  		pwake++;
> diff --git a/include/linux/wait.h b/include/linux/wait.h
> index 8aa3372f21a0..2b322a9b88a2 100644
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -221,6 +221,7 @@ void __wake_up_pollfree(struct wait_queue_head *wq_head);
>  #define wake_up_all(x)			__wake_up(x, TASK_NORMAL, 0, NULL)
>  #define wake_up_locked(x)		__wake_up_locked((x), TASK_NORMAL, 1)
>  #define wake_up_all_locked(x)		__wake_up_locked((x), TASK_NORMAL, 0)
> +#define wake_up_sync(x)			__wake_up_sync(x, TASK_NORMAL)
>  
>  #define wake_up_interruptible(x)	__wake_up(x, TASK_INTERRUPTIBLE, 1, NULL)
>  #define wake_up_interruptible_nr(x, nr)	__wake_up(x, TASK_INTERRUPTIBLE, nr, NULL)
> -- 
> 2.25.1
>