The driver_override implementation is susceptible to a race condition
when different threads are reading vs storing a different driver
override.  Add locking to avoid this race condition.

Cfr. commits 6265539776a0810b ("driver core: platform: fix race
condition with driver_override") and 9561475db680f714 ("PCI: Fix race
condition with driver_override").

Fixes: 3cf385713460eb2b ("ARM: 8256/1: driver coamba: add device binding path 
'driver_override'")
Signed-off-by: Geert Uytterhoeven <geert+rene...@glider.be>
---
 drivers/amba/bus.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/amba/bus.c b/drivers/amba/bus.c
index 6ffd778352e6d953..36c5653ced5742b7 100644
--- a/drivers/amba/bus.c
+++ b/drivers/amba/bus.c
@@ -69,8 +69,12 @@ static ssize_t driver_override_show(struct device *_dev,
                                    struct device_attribute *attr, char *buf)
 {
        struct amba_device *dev = to_amba_device(_dev);
+       ssize_t len;
 
-       return sprintf(buf, "%s\n", dev->driver_override);
+       device_lock(_dev);
+       len = sprintf(buf, "%s\n", dev->driver_override);
+       device_unlock(_dev);
+       return len;
 }
 
 static ssize_t driver_override_store(struct device *_dev,
@@ -78,7 +82,7 @@ static ssize_t driver_override_store(struct device *_dev,
                                     const char *buf, size_t count)
 {
        struct amba_device *dev = to_amba_device(_dev);
-       char *driver_override, *old = dev->driver_override, *cp;
+       char *driver_override, *old, *cp;
 
        if (count > PATH_MAX)
                return -EINVAL;
@@ -91,12 +95,15 @@ static ssize_t driver_override_store(struct device *_dev,
        if (cp)
                *cp = '\0';
 
+       device_lock(_dev);
+       old = dev->driver_override;
        if (strlen(driver_override)) {
                dev->driver_override = driver_override;
        } else {
               kfree(driver_override);
               dev->driver_override = NULL;
        }
+       device_unlock(_dev);
 
        kfree(old);
 
-- 
2.7.4

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