auxdisplay: charlcd: properly restore atomic counter on error path

[Willy Tarreau]

Commit f4757af ("staging: panel: Fix single-open policy race condition")
introduced in 3.19-rc1 attempted to fix a race condition on the open, but
failed to properly do it and used to exit without restoring the semaphore.

This results in -EBUSY being returned after the first open error until
the module is reloaded or the system restarted (ie: consecutive to a
dual open resulting in -EBUSY or to a permission error).

[ Note for stable maintainers: the code moved from drivers/misc/panel.c
  to drivers/auxdisplay/{charlcd,panel}.c during 4.12. The patch easily
  applies there (modulo the renamed atomic counter) but I can provide a
  tested backport if desired. ]

Fixes: f4757af85 # 3.19-rc1
Cc: stable@xxxxxxxxxxxxxxx
Cc: Mariusz Gorski <marius.gorski@xxxxxxxxx>
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Cc: Miguel Ojeda Sandonis <miguel.ojeda.sandonis@xxxxxxxxx>
Signed-off-by: Willy Tarreau <w@xxxxxx>
---
 drivers/auxdisplay/charlcd.c | 11 +++++++++--
 drivers/auxdisplay/panel.c   | 11 +++++++++--
 2 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/drivers/auxdisplay/charlcd.c b/drivers/auxdisplay/charlcd.c
index cfeb049..642afd8 100644
--- a/drivers/auxdisplay/charlcd.c
+++ b/drivers/auxdisplay/charlcd.c
@@ -647,18 +647,25 @@ static ssize_t charlcd_write(struct file *file, const char __user *buf,
 static int charlcd_open(struct inode *inode, struct file *file)
 {
 	struct charlcd_priv *priv = to_priv(the_charlcd);
+	int ret;
 
+	ret = -EBUSY;
 	if (!atomic_dec_and_test(&charlcd_available))
-		return -EBUSY;	/* open only once at a time */
+		goto fail;	/* open only once at a time */
 
+	ret = -EPERM;
 	if (file->f_mode & FMODE_READ)	/* device is write-only */
-		return -EPERM;
+		goto fail;
 
 	if (priv->must_clear) {
 		charlcd_clear_display(&priv->lcd);
 		priv->must_clear = false;
 	}
 	return nonseekable_open(inode, file);
+
+ fail:
+	atomic_inc(&charlcd_available);
+	return ret;
 }
 
 static int charlcd_release(struct inode *inode, struct file *file)
diff --git a/drivers/auxdisplay/panel.c b/drivers/auxdisplay/panel.c
index 7a8b8fb..c54c207 100644
--- a/drivers/auxdisplay/panel.c
+++ b/drivers/auxdisplay/panel.c
@@ -1105,14 +1105,21 @@ static ssize_t keypad_read(struct file *file,
 
 static int keypad_open(struct inode *inode, struct file *file)
 {
+	int ret;
+
+	ret = -EBUSY;
 	if (!atomic_dec_and_test(&keypad_available))
-		return -EBUSY;	/* open only once at a time */
+		goto fail;	/* open only once at a time */
 
+	ret = -EPERM;
 	if (file->f_mode & FMODE_WRITE)	/* device is read-only */
-		return -EPERM;
+		goto fail;
 
 	keypad_buflen = 0;	/* flush the buffer on opening */
 	return 0;
+ fail:
+	atomic_inc(&keypad_available);
+	return ret;
 }
 
 static int keypad_release(struct inode *inode, struct file *file)
-- 
1.7.12.1