[Xen-devel] [PATCH 17/24] xen/dev-evtchn: clean up locking in evtchn

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

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@xxxxxxxxxx>
---
 drivers/xen/evtchn.c |   37 +++++++++++++++++++++++++------------
 1 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
index 517b9ee..af03195 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;
-- 
1.6.0.6


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