powerpc/spufs: sputrace: Only enable logging on open(), prevent multiple openers

Currently, sputrace will start logging to the event buffer before the
log buffer has been open()ed. This results in a heap of "lost samples"
warnings if the sputrace file hasn't yet been opened.

Since the buffer is reset on open() anyway, there's no need to enable
logging when no-one has opened the log.

Because open clears the log, make it return EBUSY for mutliple open
calls.
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>

arch/powerpc/platforms/cell/spufs/sputrace.c

@@ -40,6 +40,7 @@ static DECLARE_WAIT_QUEUE_HEAD(sputrace_wait);
 static ktime_t sputrace_start;
 static unsigned long sputrace_head, sputrace_tail;
 static struct sputrace *sputrace_log;
+static int sputrace_logging;
 
 static int sputrace_used(void)
 {
@@ -109,24 +110,49 @@ static ssize_t sputrace_read(struct file *file, char __user *buf,
 
 static int sputrace_open(struct inode *inode, struct file *file)
 {
+	int rc;
+
 	spin_lock(&sputrace_lock);
+	if (sputrace_logging) {
+		rc = -EBUSY;
+		goto out;
+	}
+
+	sputrace_logging = 1;
 	sputrace_head = sputrace_tail = 0;
 	sputrace_start = ktime_get();
+	rc = 0;
+
+out:
 	spin_unlock(&sputrace_lock);
+	return rc;
+}
 
+static int sputrace_release(struct inode *inode, struct file *file)
+{
+	spin_lock(&sputrace_lock);
+	sputrace_logging = 0;
+	spin_unlock(&sputrace_lock);
 	return 0;
 }
 
 static const struct file_operations sputrace_fops = {
-	.owner	= THIS_MODULE,
-	.open	= sputrace_open,
-	.read	= sputrace_read,
+	.owner   = THIS_MODULE,
+	.open    = sputrace_open,
+	.read    = sputrace_read,
+	.release = sputrace_release,
 };
 
 static void sputrace_log_item(const char *name, struct spu_context *ctx,
 		struct spu *spu)
 {
 	spin_lock(&sputrace_lock);
+
+	if (!sputrace_logging) {
+		spin_unlock(&sputrace_lock);
+		return;
+	}
+
 	if (sputrace_avail() > 1) {
 		struct sputrace *t = sputrace_log + sputrace_head;