Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 15, 2009 at 10:29:56PM -0400, Gregory Haskins wrote:
> irqfd and its underlying implementation, eventfd, currently utilize
> the embedded wait-queue in eventfd for signal notification.  The nice thing
> about this design decision is that it re-uses the existing
> eventfd/wait-queue code and it generally works well....with several
> limitations.
> 
> One of the limitations is that notification callbacks are always called
> inside a spin_lock_irqsave critical section.  Another limitation is
> that it is very difficult to build a system that can recieve release
> notification without being racy.
> 
> Therefore, we introduce a new registration interface that is SRCU based
> instead of wait-queue based, and implement the internal wait-queue
> infrastructure in terms of this new interface.  We then convert irqfd
> to use this new interface instead of the existing wait-queue code.
> 
> The end result is that we now have the opportunity to run the interrupt
> injection code serially to the callback (when the signal is raised from
> process-context, at least) instead of always deferring the injection to a
> work-queue.
> 
> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> CC: Davide Libenzi <davidel@xmailserver.org>
> ---
> 
>  fs/eventfd.c            |  115 +++++++++++++++++++++++++++++++++++++++++++----
>  include/linux/eventfd.h |   30 ++++++++++++
>  virt/kvm/eventfd.c      |  114 +++++++++++++++++++++--------------------------
>  3 files changed, 188 insertions(+), 71 deletions(-)
> 
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index 72f5f8d..505d5de 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -30,8 +30,47 @@ struct eventfd_ctx {
>  	 */
>  	__u64 count;
>  	unsigned int flags;
> +	struct srcu_struct srcu;
> +	struct list_head nh;
> +	struct eventfd_notifier notifier;
>  };
>  
> +static void _eventfd_wqh_notify(struct eventfd_notifier *en)
> +{
> +	struct eventfd_ctx *ctx = container_of(en,
> +					       struct eventfd_ctx,
> +					       notifier);
> +
> +	if (waitqueue_active(&ctx->wqh))
> +		wake_up_poll(&ctx->wqh, POLLIN);
> +}
> +
> +static void _eventfd_notify(struct eventfd_ctx *ctx)
> +{
> +	struct eventfd_notifier *en;
> +	int idx;
> +
> +	idx = srcu_read_lock(&ctx->srcu);
> +
> +	/*
> +	 * The goal here is to allow the notification to be preemptible
> +	 * as often as possible.  We cannot achieve this with the basic
> +	 * wqh mechanism because it requires the wqh->lock.  Therefore
> +	 * we have an internal srcu list mechanism of which the wqh is
> +	 * a client.
> +	 *
> +	 * Not all paths will invoke this function in process context.
> +	 * Callers should check for suitable state before assuming they
> +	 * can sleep (such as with preemptible()).  Paul McKenney assures
> +	 * me that srcu_read_lock is compatible with in-atomic, as long as
> +	 * the code within the critical section is also compatible.
> +	 */
> +	list_for_each_entry_rcu(en, &ctx->nh, list)
> +		en->ops->signal(en);
> +
> +	srcu_read_unlock(&ctx->srcu, idx);
> +}
> +
>  /*
>   * Adds "n" to the eventfd counter "count". Returns "n" in case of
>   * success, or a value lower then "n" in case of coutner overflow.

This is ugly, isn't it? With CONFIG_PREEMPT=no preemptible() is always false.

Further, to do useful things it might not be enough that you can sleep:
with iofd you also want to access current task with e.g. copy from user.

Here's an idea: let's pass a flag to ->signal, along the lines of
signal_is_task, that tells us that it is safe to use current, and add
eventfd_signal_task() which is the same as eventfd_signal but lets everyone
know that it's safe to both sleep and use current->mm.

Makes sense?

-- 
MST