9p: Convert semaphore to spinlock for p9_idpool

When booting from v9fs, down_interruptible in p9_idpool_get() triggered a BUG
as it was being called with IRQs disabled.  A spinlock seems like the right
thing to be using since the idr functions go out of their way not to sleep.

This patch eliminates the BUG by converting the semaphore to a spinlock.
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Acked-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/util.c

@@ -33,7 +33,7 @@
 #include <net/9p/9p.h>
 
 struct p9_idpool {
-	struct semaphore lock;
+	spinlock_t lock;
 	struct idr pool;
 };
 
@@ -45,7 +45,7 @@ struct p9_idpool *p9_idpool_create(void)
 	if (!p)
 		return ERR_PTR(-ENOMEM);
 
-	init_MUTEX(&p->lock);
+	spin_lock_init(&p->lock);
 	idr_init(&p->pool);
 
 	return p;
@@ -71,19 +71,17 @@ int p9_idpool_get(struct p9_idpool *p)
 {
 	int i = 0;
 	int error;
+	unsigned int flags;
 
 retry:
 	if (idr_pre_get(&p->pool, GFP_KERNEL) == 0)
 		return 0;
 
-	if (down_interruptible(&p->lock) == -EINTR) {
-		P9_EPRINTK(KERN_WARNING, "Interrupted while locking\n");
-		return -1;
-	}
+	spin_lock_irqsave(&p->lock, flags);
 
 	/* no need to store exactly p, we just need something non-null */
 	error = idr_get_new(&p->pool, p, &i);
-	up(&p->lock);
+	spin_unlock_irqrestore(&p->lock, flags);
 
 	if (error == -EAGAIN)
 		goto retry;
@@ -104,12 +102,10 @@ EXPORT_SYMBOL(p9_idpool_get);
 
 void p9_idpool_put(int id, struct p9_idpool *p)
 {
-	if (down_interruptible(&p->lock) == -EINTR) {
-		P9_EPRINTK(KERN_WARNING, "Interrupted while locking\n");
-		return;
-	}
+	unsigned int flags;
+	spin_lock_irqsave(&p->lock, flags);
 	idr_remove(&p->pool, id);
-	up(&p->lock);
+	spin_unlock_irqrestore(&p->lock, flags);
 }
 EXPORT_SYMBOL(p9_idpool_put);