[PATCH] iio: trigger: free the trigger ID after device_del()

[Changyul Lee]

iio_trigger_unregister() returns the trigger ID to iio_trigger_ida
before device_del() removes the "trigger<N>" device. A trigger
registered in between can get the same ID, and its device_add() fails:

  sysfs: cannot create duplicate filename '/bus/iio/devices/trigger0'

For example, adding a sysfs trigger while an hrtimer trigger is being
removed through configfs, or the other way around, hits this because the
two paths do not share a lock.

Free the ID after device_del(), as the error path of
iio_trigger_register() already does, and drop the "Possible issue in
here" comment.

Fixes: 1637db44413e ("Staging: IIO: Trigger support added to core.")
Signed-off-by: Changyul Lee <lcy8047@xxxxxxxxx>
---
Found with a static analysis tool we are developing.

Reproduced in QEMU (x86_64) with two threads running for 20 seconds.
One writes to add_trigger and remove_trigger of iio_sysfs_trigger, the
other creates and removes an hrtimer trigger in configfs. Without this
patch, device_add() failed with the error above 160 to 1148 times per
run in three runs. With it, it never failed.

 drivers/iio/industrialio-trigger.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/iio/industrialio-trigger.c b/drivers/iio/industrialio-trigger.c
index 76926fee0f84..206ede2c9b3c 100644
--- a/drivers/iio/industrialio-trigger.c
+++ b/drivers/iio/industrialio-trigger.c
@@ -105,9 +105,8 @@ void iio_trigger_unregister(struct iio_trigger *trig_info)
 	scoped_guard(mutex, &iio_trigger_list_lock)
 		list_del(&trig_info->list);
 
-	ida_free(&iio_trigger_ida, trig_info->id);
-	/* Possible issue in here */
 	device_del(&trig_info->dev);
+	ida_free(&iio_trigger_ida, trig_info->id);
 }
 EXPORT_SYMBOL(iio_trigger_unregister);
 

base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4
-- 
2.43.0