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Subject[KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface
Date
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.
@@ -51,10 +90,10 @@ int eventfd_signal(struct file *file, int n)
if (ULLONG_MAX - ctx->count < n)
n = (int) (ULLONG_MAX - ctx->count);
ctx->count += n;
- if (waitqueue_active(&ctx->wqh))
- wake_up_locked_poll(&ctx->wqh, POLLIN);
spin_unlock_irqrestore(&ctx->wqh.lock, flags);

+ _eventfd_notify(ctx);
+
return n;
}
EXPORT_SYMBOL_GPL(eventfd_signal);
@@ -62,13 +101,21 @@ EXPORT_SYMBOL_GPL(eventfd_signal);
static int eventfd_release(struct inode *inode, struct file *file)
{
struct eventfd_ctx *ctx = file->private_data;
+ struct eventfd_notifier *en, *tmp;

/*
* No need to hold the lock here, since we are on the file cleanup
- * path and the ones still attached to the wait queue will be
- * serialized by wake_up_locked_poll().
+ * path
*/
- wake_up_locked_poll(&ctx->wqh, POLLHUP);
+ list_for_each_entry_safe(en, tmp, &ctx->nh, list) {
+ list_del(&en->list);
+ if (en->ops->release)
+ en->ops->release(en);
+ }
+
+ synchronize_srcu(&ctx->srcu);
+ cleanup_srcu_struct(&ctx->srcu);
+
kfree(ctx);
return 0;
}
@@ -176,13 +223,13 @@ static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t c
__remove_wait_queue(&ctx->wqh, &wait);
__set_current_state(TASK_RUNNING);
}
- if (likely(res > 0)) {
+ if (likely(res > 0))
ctx->count += ucnt;
- if (waitqueue_active(&ctx->wqh))
- wake_up_locked_poll(&ctx->wqh, POLLIN);
- }
spin_unlock_irq(&ctx->wqh.lock);

+ if (likely(res > 0))
+ _eventfd_notify(ctx);
+
return res;
}

@@ -209,6 +256,50 @@ struct file *eventfd_fget(int fd)
}
EXPORT_SYMBOL_GPL(eventfd_fget);

+static int _eventfd_notifier_register(struct eventfd_ctx *ctx,
+ struct eventfd_notifier *en)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&ctx->wqh.lock, flags);
+ list_add_tail_rcu(&en->list, &ctx->nh);
+ spin_unlock_irqrestore(&ctx->wqh.lock, flags);
+
+ return 0;
+}
+
+int eventfd_notifier_register(struct file *file, struct eventfd_notifier *en)
+{
+ struct eventfd_ctx *ctx = file->private_data;
+
+ if (file->f_op != &eventfd_fops)
+ return -EINVAL;
+
+ return _eventfd_notifier_register(ctx, en);
+}
+EXPORT_SYMBOL_GPL(eventfd_notifier_register);
+
+int eventfd_notifier_unregister(struct file *file, struct eventfd_notifier *en)
+{
+ struct eventfd_ctx *ctx = file->private_data;
+
+ if (file->f_op != &eventfd_fops)
+ return -EINVAL;
+
+ spin_lock_irq(&ctx->wqh.lock);
+ list_del_rcu(&en->list);
+ spin_unlock_irq(&ctx->wqh.lock);
+
+ synchronize_srcu(&ctx->srcu);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(eventfd_notifier_unregister);
+
+static const struct eventfd_notifier_ops eventfd_wqh_ops = {
+ .signal = _eventfd_wqh_notify,
+};
+
SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
{
int fd;
@@ -229,6 +320,12 @@ SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
ctx->count = count;
ctx->flags = flags;

+ init_srcu_struct(&ctx->srcu);
+ INIT_LIST_HEAD(&ctx->nh);
+
+ eventfd_notifier_init(&ctx->notifier, &eventfd_wqh_ops);
+ _eventfd_notifier_register(ctx, &ctx->notifier);
+
/*
* When we call this, the initialization must be complete, since
* anon_inode_getfd() will install the fd.
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index f45a8ae..0218cf6 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -8,6 +8,28 @@
#ifndef _LINUX_EVENTFD_H
#define _LINUX_EVENTFD_H

+#include <linux/list.h>
+
+struct eventfd_notifier;
+
+struct eventfd_notifier_ops {
+ void (*signal)(struct eventfd_notifier *en);
+ void (*release)(struct eventfd_notifier *en);
+};
+
+struct eventfd_notifier {
+ struct list_head list;
+ const struct eventfd_notifier_ops *ops;
+};
+
+static inline void eventfd_notifier_init(struct eventfd_notifier *en,
+ const struct eventfd_notifier_ops *ops)
+{
+ memset(en, 0, sizeof(*en));
+ INIT_LIST_HEAD(&en->list);
+ en->ops = ops;
+}
+
#ifdef CONFIG_EVENTFD

/* For O_CLOEXEC and O_NONBLOCK */
@@ -29,12 +51,20 @@

struct file *eventfd_fget(int fd);
int eventfd_signal(struct file *file, int n);
+int eventfd_notifier_register(struct file *file, struct eventfd_notifier *en);
+int eventfd_notifier_unregister(struct file *file, struct eventfd_notifier *en);

#else /* CONFIG_EVENTFD */

#define eventfd_fget(fd) ERR_PTR(-ENOSYS)
static inline int eventfd_signal(struct file *file, int n)
{ return 0; }
+static inline int eventfd_notifier_register(struct file *file,
+ struct eventfd_notifier *en)
+{ return -ENOENT; }
+static inline int eventfd_notifier_unregister(struct file *file,
+ struct eventfd_notifier *en)
+{ return -ENOENT; }

#endif /* CONFIG_EVENTFD */

diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 2c8028c..efc3d77 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -43,33 +43,11 @@ struct _irqfd {
struct kvm *kvm;
int gsi;
struct list_head list;
- poll_table pt;
- wait_queue_head_t *wqh;
- wait_queue_t wait;
struct work_struct inject;
+ struct eventfd_notifier notifier;
};

static void
-irqfd_inject(struct work_struct *work)
-{
- struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
- struct kvm *kvm;
- int idx;
-
- idx = srcu_read_lock(&irqfd->srcu);
-
- kvm = rcu_dereference(irqfd->kvm);
- if (kvm) {
- mutex_lock(&kvm->irq_lock);
- kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
- kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
- mutex_unlock(&kvm->irq_lock);
- }
-
- srcu_read_unlock(&irqfd->srcu, idx);
-}
-
-static void
irqfd_disconnect(struct _irqfd *irqfd)
{
struct kvm *kvm;
@@ -97,47 +75,67 @@ irqfd_disconnect(struct _irqfd *irqfd)
kvm_put_kvm(kvm);
}

-static int
-irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
+static void
+_irqfd_inject(struct _irqfd *irqfd)
{
- struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait);
- unsigned long flags = (unsigned long)key;
-
- if (flags & POLLIN)
- /*
- * The POLLIN wake_up is called with interrupts disabled.
- * Therefore we need to defer the IRQ injection until later
- * since we need to acquire the kvm->lock to do so.
- */
- schedule_work(&irqfd->inject);
+ struct kvm *kvm;
+ int idx;

- if (flags & POLLHUP) {
- /*
- * The POLLHUP is called unlocked, so it theoretically should
- * be safe to remove ourselves from the wqh using the locked
- * variant of remove_wait_queue()
- */
- remove_wait_queue(irqfd->wqh, &irqfd->wait);
- flush_work(&irqfd->inject);
- irqfd_disconnect(irqfd);
+ idx = srcu_read_lock(&irqfd->srcu);

- cleanup_srcu_struct(&irqfd->srcu);
- kfree(irqfd);
+ kvm = rcu_dereference(irqfd->kvm);
+ if (kvm) {
+ mutex_lock(&kvm->irq_lock);
+ kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
+ kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
+ mutex_unlock(&kvm->irq_lock);
}

- return 0;
+ srcu_read_unlock(&irqfd->srcu, idx);
+}
+
+static void
+irqfd_inject(struct work_struct *work)
+{
+ struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
+
+ _irqfd_inject(irqfd);
+}
+
+static void
+irqfd_eventfd_signal(struct eventfd_notifier *notifier)
+{
+ struct _irqfd *irqfd = container_of(notifier, struct _irqfd, notifier);
+
+ /*
+ * Our signal may or may not be called from atomic context. We can
+ * detect if this can safely sleep by checking preemptible() on
+ * CONFIG_PREEMPT kernels. CONFIG_PREEMPT=n, or in_atomic() will fail
+ * this check and therefore defer the injection via a work-queue
+ */
+ if (preemptible())
+ _irqfd_inject(irqfd);
+ else
+ schedule_work(&irqfd->inject);
}

static void
-irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
- poll_table *pt)
+irqfd_eventfd_release(struct eventfd_notifier *notifier)
{
- struct _irqfd *irqfd = container_of(pt, struct _irqfd, pt);
+ struct _irqfd *irqfd = container_of(notifier, struct _irqfd, notifier);

- irqfd->wqh = wqh;
- add_wait_queue(wqh, &irqfd->wait);
+ flush_work(&irqfd->inject);
+ irqfd_disconnect(irqfd);
+
+ cleanup_srcu_struct(&irqfd->srcu);
+ kfree(irqfd);
}

+static const struct eventfd_notifier_ops irqfd_notifier_ops = {
+ .signal = irqfd_eventfd_signal,
+ .release = irqfd_eventfd_release,
+};
+
int
kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
{
@@ -165,14 +163,9 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
goto fail;
}

- /*
- * Install our own custom wake-up handling so we are notified via
- * a callback whenever someone signals the underlying eventfd
- */
- init_waitqueue_func_entry(&irqfd->wait, irqfd_wakeup);
- init_poll_funcptr(&irqfd->pt, irqfd_ptable_queue_proc);
+ eventfd_notifier_init(&irqfd->notifier, &irqfd_notifier_ops);

- ret = file->f_op->poll(file, &irqfd->pt);
+ ret = eventfd_notifier_register(file, &irqfd->notifier);
if (ret < 0)
goto fail;

@@ -191,9 +184,6 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
return 0;

fail:
- if (irqfd->wqh)
- remove_wait_queue(irqfd->wqh, &irqfd->wait);
-
if (file && !IS_ERR(file))
fput(file);



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