[Xen-devel] [PATCH 1/3] xen/event: Add reference counting to event channels

[Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>]

Event channels exposed to userspace by the evtchn module may be used by
other modules in an asynchronous manner, which requires that reference
counting be used to prevent the event channel from being closed before
the signals are delivered.

The reference count on new event channels defaults to -1 which indicates
the event channel is not referenced outside the kernel; evtchn_get fails
if called on such an event channel. The event channels made visible to
userspace by evtchn have a normal reference count.

Signed-off-by: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
---
 drivers/xen/events.c |   62 +++++++++++++++++++++++++++++++++++++++++++++++++-
 drivers/xen/evtchn.c |    2 +-
 include/xen/events.h |    7 +++++
 3 files changed, 69 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/events.c b/drivers/xen/events.c
index 073c11d..6054580 100644
--- a/drivers/xen/events.c
+++ b/drivers/xen/events.c
@@ -87,6 +87,7 @@ enum xen_irq_type {
  */
 struct irq_info {
        struct list_head list;
+       atomic_t refcnt;
        enum xen_irq_type type; /* type */
        unsigned irq;
        unsigned short evtchn;  /* event channel */
@@ -406,6 +407,7 @@ static void xen_irq_init(unsigned irq)
                panic("Unable to allocate metadata for IRQ%d\n", irq);
 
        info->type = IRQT_UNBOUND;
+       atomic_set(&info->refcnt, -1);
 
        irq_set_handler_data(irq, info);
 
@@ -469,6 +471,8 @@ static void xen_free_irq(unsigned irq)
 
        irq_set_handler_data(irq, NULL);
 
+       WARN_ON(atomic_read(&info->refcnt) > 0);
+
        kfree(info);
 
        /* Legacy IRQ descriptors are managed by the arch. */
@@ -637,7 +641,7 @@ int xen_bind_pirq_gsi_to_irq(unsigned gsi,
        if (irq != -1) {
                printk(KERN_INFO "xen_map_pirq_gsi: returning irq %d for gsi 
%u\n",
                       irq, gsi);
-               goto out;       /* XXX need refcount? */
+               goto out;
        }
 
        irq = xen_allocate_irq_gsi(gsi);
@@ -939,6 +943,10 @@ static void unbind_from_irq(unsigned int irq)
 {
        struct evtchn_close close;
        int evtchn = evtchn_from_irq(irq);
+       struct irq_info *info = irq_get_handler_data(irq);
+
+       if (atomic_read(&info->refcnt) > 0 && 
!atomic_dec_and_test(&info->refcnt))
+               return;
 
        mutex_lock(&irq_mapping_update_lock);
 
@@ -1065,6 +1073,58 @@ void unbind_from_irqhandler(unsigned int irq, void 
*dev_id)
 }
 EXPORT_SYMBOL_GPL(unbind_from_irqhandler);
 
+int evtchn_make_refcounted(unsigned int evtchn)
+{
+       int irq = evtchn_to_irq[evtchn];
+       struct irq_info *info;
+
+       if (irq == -1)
+               return -ENOENT;
+
+       info = irq_get_handler_data(irq);
+
+       if (!info)
+               return -ENOENT;
+
+       WARN_ON(atomic_read(&info->refcnt) != -1);
+
+       atomic_set(&info->refcnt, 1);
+
+       return 0;
+}
+EXPORT_SYMBOL_GPL(evtchn_make_refcounted);
+
+int evtchn_get(unsigned int evtchn)
+{
+       int irq = evtchn_to_irq[evtchn];
+       struct irq_info *info;
+
+       if (irq == -1)
+               return -ENOENT;
+
+       info = irq_get_handler_data(irq);
+
+       if (!info)
+               return -ENOENT;
+
+       if (atomic_read(&info->refcnt) < 0)
+               return -EINVAL;
+
+       atomic_inc(&info->refcnt);
+
+       return 0;
+}
+EXPORT_SYMBOL_GPL(evtchn_get);
+
+void evtchn_put(unsigned int evtchn)
+{
+       int irq = evtchn_to_irq[evtchn];
+       if (WARN_ON(irq == -1))
+               return;
+       unbind_from_irq(irq);
+}
+EXPORT_SYMBOL_GPL(evtchn_put);
+
 void xen_send_IPI_one(unsigned int cpu, enum ipi_vector vector)
 {
        int irq = per_cpu(ipi_to_irq, cpu)[vector];
diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
index dbc13e9..b1f60a0 100644
--- a/drivers/xen/evtchn.c
+++ b/drivers/xen/evtchn.c
@@ -268,7 +268,7 @@ static int evtchn_bind_to_user(struct per_user_data *u, int 
port)
        rc = bind_evtchn_to_irqhandler(port, evtchn_interrupt, IRQF_DISABLED,
                                       u->name, (void *)(unsigned long)port);
        if (rc >= 0)
-               rc = 0;
+               rc = evtchn_make_refcounted(port);
 
        return rc;
 }
diff --git a/include/xen/events.h b/include/xen/events.h
index d287997..0f77370 100644
--- a/include/xen/events.h
+++ b/include/xen/events.h
@@ -37,6 +37,13 @@ int bind_interdomain_evtchn_to_irqhandler(unsigned int 
remote_domain,
  */
 void unbind_from_irqhandler(unsigned int irq, void *dev_id);
 
+/*
+ * Allow extra references to event channels exposed to userspace by evtchn
+ */
+int evtchn_make_refcounted(unsigned int evtchn);
+int evtchn_get(unsigned int evtchn);
+void evtchn_put(unsigned int evtchn);
+
 void xen_send_IPI_one(unsigned int cpu, enum ipi_vector vector);
 int resend_irq_on_evtchn(unsigned int irq);
 void rebind_evtchn_irq(int evtchn, int irq);
-- 
1.7.6.4


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