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


    --
    MST


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