coresight: tmc: getting rid of multiple read access

Allowing multiple readers to access the trace data simultaniously
via sysFS provides no shortage of opportunity for race condition,
mandates two variable to be maintained (drvdata::read_count and
drvdata::reading), makes the code complex and provide little
advantages, if any.

This patch streamlines the read process by restricting trace data
access to a single user.  That way drvdata::read_count can
be eliminated and race conditions (along with faulty error handling)
in function tmc_open() and tmc_release() eliminated.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 9 insertions, 15 deletions

diff --git a/drivers/hwtracing/coresight/coresight-tmc.c b/drivers/hwtracing/coresight/coresight-tmc.c
index e8e12a9b917a..ae7525a2b94a 100644
--- a/drivers/hwtracing/coresight/coresight-tmc.c
+++ b/drivers/hwtracing/coresight/coresight-tmc.c
@@ -95,7 +95,7 @@ static int tmc_read_prepare(struct tmc_drvdata *drvdata)
 	return ret;
 }
 
-static void tmc_read_unprepare(struct tmc_drvdata *drvdata)
+static int tmc_read_unprepare(struct tmc_drvdata *drvdata)
 {
 	int ret = 0;
 
@@ -113,21 +113,20 @@ static void tmc_read_unprepare(struct tmc_drvdata *drvdata)
 
 	if (!ret)
 		dev_info(drvdata->dev, "TMC read end\n");
+
+	return ret;
 }
 
 static int tmc_open(struct inode *inode, struct file *file)
 {
+	int ret;
 	struct tmc_drvdata *drvdata = container_of(file->private_data,
 						   struct tmc_drvdata, miscdev);
-	int ret = 0;
-
-	if (drvdata->read_count++)
-		goto out;
 
 	ret = tmc_read_prepare(drvdata);
 	if (ret)
 		return ret;
-out:
+
 	nonseekable_open(inode, file);
 
 	dev_dbg(drvdata->dev, "%s: successfully opened\n", __func__);
@@ -167,19 +166,14 @@ static ssize_t tmc_read(struct file *file, char __user *data, size_t len,
 
 static int tmc_release(struct inode *inode, struct file *file)
 {
+	int ret;
 	struct tmc_drvdata *drvdata = container_of(file->private_data,
 						   struct tmc_drvdata, miscdev);
 
-	if (--drvdata->read_count) {
-		if (drvdata->read_count < 0) {
-			dev_err(drvdata->dev, "mismatched close\n");
-			drvdata->read_count = 0;
-		}
-		goto out;
-	}
+	ret = tmc_read_unprepare(drvdata);
+	if (ret)
+		return ret;
 
-	tmc_read_unprepare(drvdata);
-out:
 	dev_dbg(drvdata->dev, "%s: released\n", __func__);
 	return 0;
 }