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

Gregory Haskins
2009-06-16T14:48:49Z

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Michael S. Tsirkin wrote:
> On Tue, Jun 16, 2009 at 10:11:08AM -0400, Gregory Haskins wrote:
>
>> Michael S. Tsirkin wrote:
>>
>>> 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?
>>>
>>>
>> It does make sense, yes. What I am not clear on is how would eventfd
>> detect this state such as to populate such flags, and why cant the
>> ->signal() CB do the same?
>>
>> Thanks Michael,
>> -Greg
>>
>>
>
> eventfd can't detect this state. But the callers know in what context they are.
> So the *caller* of eventfd_signal_task makes sure of this: if you are in a task,
> you can call eventfd_signal_task() if not, you must call eventfd_signal.
>
>
>
Hmm, this is an interesting idea, but I think it would be problematic in
real-world applications for the long-term. For instance, the -rt tree
and irq-threads .config option in the process of merging into mainline
changes context types for established code. Therefore, what might be
"hardirq/softirq" logic today may execute in a kthread tomorrow. I
think its dangerous to try to solve the problem with caller provided
info: the caller may be ignorant of its true state. IMO, the ideal
solution needs to be something we can detect at run-time.

Thanks Michael,
-Greg

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