xen/dev-evtchn: clean up locking in evtchn

Define a new per_user_data mutex to serialize bind/unbind operations
to prevent them from racing with each other.  Fix error returns
and don't do a bind while holding a spinlock.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

1 files changed, 25 insertions, 12 deletions

diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
index 517b9ee63e18..af031950f9b1 100644
--- a/drivers/xen/evtchn.c
+++ b/drivers/xen/evtchn.c
@@ -54,6 +54,8 @@
 #include <asm/xen/hypervisor.h>
 
 struct per_user_data {
+        struct mutex bind_mutex; /* serialize bind/unbind operations */
+
         /* Notification ring, accessed via /dev/xen/evtchn. */
 #define EVTCHN_RING_SIZE     (PAGE_SIZE / sizeof(evtchn_port_t))
 #define EVTCHN_RING_MASK(_i) ((_i)&(EVTCHN_RING_SIZE-1))
@@ -69,7 +71,7 @@ struct per_user_data {
 
 /* Who's bound to each port? */
 static struct per_user_data *port_user[NR_EVENT_CHANNELS];
-static DEFINE_SPINLOCK(port_user_lock);
+static DEFINE_SPINLOCK(port_user_lock); /* protects port_user[] and ring_prod */
 
 irqreturn_t evtchn_interrupt(int irq, void *data)
 {
@@ -210,22 +212,24 @@ static ssize_t evtchn_write(struct file *file, const char __user *buf,
 
 static int evtchn_bind_to_user(struct per_user_data *u, int port)
 {
-        int irq;
         int rc = 0;
 
-        spin_lock_irq(&port_user_lock);
-
+        /*
+         * Ports are never reused, so every caller should pass in a
+         * unique port.
+         *
+         * (Locking not necessary because we haven't registered the
+         * interrupt handler yet, and our caller has already
+         * serialized bind operations.)
+         */
         BUG_ON(port_user[port] != NULL);
-
-        irq = bind_evtchn_to_irqhandler(port, evtchn_interrupt, IRQF_DISABLED,
-                                        u->name, (void *)(unsigned long)port);
-        if (rc < 0)
-                goto fail;
-
         port_user[port] = u;
 
-fail:
-        spin_unlock_irq(&port_user_lock);
+        rc = bind_evtchn_to_irqhandler(port, evtchn_interrupt, IRQF_DISABLED,
+                                       u->name, (void *)(unsigned long)port);
+        if (rc >= 0)
+                rc = 0;
+
         return rc;
 }
 
@@ -234,6 +238,10 @@ static void evtchn_unbind_from_user(struct per_user_data *u, int port)
         int irq = irq_from_evtchn(port);
 
         unbind_from_irqhandler(irq, (void *)(unsigned long)port);
+
+        /* make sure we unbind the irq handler before clearing the port */
+        barrier();
+
         port_user[port] = NULL;
 }
 
@@ -244,6 +252,9 @@ static long evtchn_ioctl(struct file *file,
         struct per_user_data *u = file->private_data;
         void __user *uarg = (void __user *) arg;
 
+        /* Prevent bind from racing with unbind */
+        mutex_lock(&u->bind_mutex);
+
         switch (cmd) {
         case IOCTL_EVTCHN_BIND_VIRQ: {
                 struct ioctl_evtchn_bind_virq bind;
@@ -368,6 +379,7 @@ static long evtchn_ioctl(struct file *file,
                 rc = -ENOSYS;
                 break;
         }
+        mutex_unlock(&u->bind_mutex);
 
         return rc;
 }
@@ -414,6 +426,7 @@ static int evtchn_open(struct inode *inode, struct file *filp)
                 return -ENOMEM;
         }
 
+        mutex_init(&u->bind_mutex);
         mutex_init(&u->ring_cons_mutex);
 
         filp->private_data = u;