Patch "base: make module_create_drivers_dir race-free" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

base: make module_create_drivers_dir race-free

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
base-make-module_create_drivers_dir-race-free.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jiri Slaby <[email protected]>
Date: Fri, 10 Jun 2016 10:54:32 +0200
Subject: base: make module_create_drivers_dir race-free

From: Jiri Slaby <[email protected]>

commit 7e1b1fc4dabd6ec8e28baa0708866e13fa93c9b3 upstream.

Modules which register drivers via standard path (driver_register) in parallel can cause a warning:

WARNING: CPU: 2 PID: 3492 at ../fs/sysfs/dir.c:31 sysfs_warn_dup+0x62/0x80
sysfs: cannot create duplicate filename '/module/saa7146/drivers'
Modules linked in: hexium_gemini(+) mxb(+) ...
...
Call Trace:
...
 [<ffffffff812e63a2>] sysfs_warn_dup+0x62/0x80
 [<ffffffff812e6487>] sysfs_create_dir_ns+0x77/0x90
 [<ffffffff8140f2c4>] kobject_add_internal+0xb4/0x340
 [<ffffffff8140f5b8>] kobject_add+0x68/0xb0
 [<ffffffff8140f631>] kobject_create_and_add+0x31/0x70
 [<ffffffff8157a703>] module_add_driver+0xc3/0xd0
 [<ffffffff8155e5d4>] bus_add_driver+0x154/0x280
 [<ffffffff815604c0>] driver_register+0x60/0xe0
 [<ffffffff8145bed0>] __pci_register_driver+0x60/0x70
 [<ffffffffa0273e14>] saa7146_register_extension+0x64/0x90 [saa7146]
 [<ffffffffa0033011>] hexium_init_module+0x11/0x1000 [hexium_gemini]

...

As can be (mostly) seen, driver_register causes this call sequence:
-> bus_add_driver

    -> module_add_driver
      -> module_create_drivers_dir

The last one creates "drivers" directory in /sys/module/<...>. When this is done in parallel, the directory is attempted to be created twice at the same time.

This can be easily reproduced by loading mxb and hexium_gemini in parallel:
while :; do
modprobe mxb &
modprobe hexium_gemini
wait
rmmod mxb hexium_gemini saa7146_vv saa7146
done

saa7146 calls pci_register_driver for both mxb and hexium_gemini, which means /sys/module/saa7146/drivers is to be created for both of them.

Fix this by a new mutex in module_create_drivers_dir which makes the test-and-create "drivers" dir atomic.

I inverted the condition and removed 'return' to avoid multiple unlocks or a goto.

---
 drivers/base/module.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

@@ -24,10 +24,12 @@ static char *make_driver_name(struct dev
 
 static void module_create_drivers_dir(struct module_kobject *mk)
 {
-	if (!mk || mk->drivers_dir)
-		return;
+	static DEFINE_MUTEX(drivers_dir_mutex);
 
-	mk->drivers_dir = kobject_create_and_add("drivers", &mk->kobj);
+	mutex_lock(&drivers_dir_mutex);
+	if (mk && !mk->drivers_dir)
+		mk->drivers_dir = kobject_create_and_add("drivers", &mk->kobj);
+	mutex_unlock(&drivers_dir_mutex);
 }
 
 void module_add_driver(struct module *mod, struct device_driver *drv)

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/base-make-module_create_drivers_dir-race-free.patch