Re: [PATCH] eventfd: document lockless access in eventfd_poll

[Andrea Arcangeli <aarcange@redhat.com>]

On Wed, Mar 02, 2016 at 11:01:58PM +0100, Paolo Bonzini wrote:
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index 8d0c0df01854..dbbbe203f82b 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -121,8 +121,46 @@ static unsigned int eventfd_poll(struct file *file, poll_table *wait)
>  	u64 count;
>  
>  	poll_wait(file, &ctx->wqh, wait);
> -	smp_rmb();
> -	count = ctx->count;
> +
> +	/*
> +	 * All writes to ctx->count occur within ctx->wqh.lock.  This read
> +	 * can be done outside ctx->wqh.lock because we know that poll_wait
> +	 * takes that lock (through add_wait_queue) if our caller will sleep.
> +	 *
> +	 * The read _can_ therefore seep into add_wait_queue's critical
> +	 * section, but cannot move above it!  add_wait_queue's spin_lock acts
> +	 * as an acquire barrier and ensures that the read be ordered properly
> +	 * against the writes.  The following CAN happen and is safe:
> +	 *
> +	 *     poll                               write
> +	 *     -----------------                  ------------
> +	 *     lock ctx->wqh.lock (in poll_wait)
> +	 *     count = ctx->count
> +	 *     __add_wait_queue
> +	 *     unlock ctx->wqh.lock
> +	 *                                        lock ctx->qwh.lock
> +	 *                                        ctx->count += n
> +	 *                                        if (waitqueue_active)
> +	 *                                          wake_up_locked_poll
> +	 *                                        unlock ctx->qwh.lock
> +	 *     eventfd_poll returns 0
> +	 *
> +	 * but the following, which would miss a wakeup, cannot happen:
> +	 *
> +	 *     poll                               write
> +	 *     -----------------                  ------------
> +	 *     count = ctx->count (INVALID!)
> +	 *                                        lock ctx->qwh.lock
> +	 *                                        ctx->count += n
> +	 *                                        **waitqueue_active is false**
> +	 *                                        **no wake_up_locked_poll!**
> +	 *                                        unlock ctx->qwh.lock
> +	 *     lock ctx->wqh.lock (in poll_wait)
> +	 *     __add_wait_queue
> +	 *     unlock ctx->wqh.lock
> +	 *     eventfd_poll returns 0
> +	 */
> +	count = READ_ONCE(ctx->count);
>  
>  	if (count > 0)
>  		events |= POLLIN;

Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>