3.16.72-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Fabrice Gasnier fabrice.gasnier@st.com
commit 7f75591fc5a123929a29636834d1bcb8b5c9fee3 upstream.
This fixes a possible circular locking dependency detected warning seen with: - CONFIG_PROVE_LOCKING=y - consumer/provider IIO devices (ex: "voltage-divider" consumer of "adc")
When using the IIO consumer interface, e.g. iio_channel_get(), the consumer device will likely call iio_read_channel_raw() or similar that rely on 'info_exist_lock' mutex.
typically: ... mutex_lock(&chan->indio_dev->info_exist_lock); if (chan->indio_dev->info == NULL) { ret = -ENODEV; goto err_unlock; } ret = do_some_ops() err_unlock: mutex_unlock(&chan->indio_dev->info_exist_lock); return ret; ...
Same mutex is also hold in iio_device_unregister().
The following deadlock warning happens when: - the consumer device has called an API like iio_read_channel_raw() at least once. - the consumer driver is unregistered, removed (unbind from sysfs)
====================================================== WARNING: possible circular locking dependency detected 4.19.24 #577 Not tainted ------------------------------------------------------ sh/372 is trying to acquire lock: (kn->count#30){++++}, at: kernfs_remove_by_name_ns+0x3c/0x84
but task is already holding lock: (&dev->info_exist_lock){+.+.}, at: iio_device_unregister+0x18/0x60
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&dev->info_exist_lock){+.+.}: __mutex_lock+0x70/0xa3c mutex_lock_nested+0x1c/0x24 iio_read_channel_raw+0x1c/0x60 iio_read_channel_info+0xa8/0xb0 dev_attr_show+0x1c/0x48 sysfs_kf_seq_show+0x84/0xec seq_read+0x154/0x528 __vfs_read+0x2c/0x15c vfs_read+0x8c/0x110 ksys_read+0x4c/0xac ret_fast_syscall+0x0/0x28 0xbedefb60
-> #0 (kn->count#30){++++}: lock_acquire+0xd8/0x268 __kernfs_remove+0x288/0x374 kernfs_remove_by_name_ns+0x3c/0x84 remove_files+0x34/0x78 sysfs_remove_group+0x40/0x9c sysfs_remove_groups+0x24/0x34 device_remove_attrs+0x38/0x64 device_del+0x11c/0x360 cdev_device_del+0x14/0x2c iio_device_unregister+0x24/0x60 release_nodes+0x1bc/0x200 device_release_driver_internal+0x1a0/0x230 unbind_store+0x80/0x130 kernfs_fop_write+0x100/0x1e4 __vfs_write+0x2c/0x160 vfs_write+0xa4/0x17c ksys_write+0x4c/0xac ret_fast_syscall+0x0/0x28 0xbe906840
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1 ---- ---- lock(&dev->info_exist_lock); lock(kn->count#30); lock(&dev->info_exist_lock); lock(kn->count#30);
*** DEADLOCK *** ...
cdev_device_del() can be called without holding the lock. It should be safe as info_exist_lock prevents kernelspace consumers to use the exported routines during/after provider removal. cdev_device_del() is for userspace.
Help to reproduce: See example: Documentation/devicetree/bindings/iio/afe/voltage-divider.txt sysv { compatible = "voltage-divider"; io-channels = <&adc 0>; output-ohms = <22>; full-ohms = <222>; };
First, go to iio:deviceX for the "voltage-divider", do one read: $ cd /sys/bus/iio/devices/iio:deviceX $ cat in_voltage0_raw
Then, unbind the consumer driver. It triggers above deadlock warning. $ cd /sys/bus/platform/drivers/iio-rescale/ $ echo sysv > unbind
Note I don't actually expect stable will pick this up all the way back into IIO being in staging, but if's probably valid that far back.
Signed-off-by: Fabrice Gasnier fabrice.gasnier@st.com Fixes: ac917a81117c ("staging:iio:core set the iio_dev.info pointer to null on unregister") Signed-off-by: Jonathan Cameron Jonathan.Cameron@huawei.com [bwh: Backported to 3.16: adjust context] Signed-off-by: Ben Hutchings ben@decadent.org.uk --- drivers/iio/industrialio-core.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
--- a/drivers/iio/industrialio-core.c +++ b/drivers/iio/industrialio-core.c @@ -1195,12 +1195,12 @@ EXPORT_SYMBOL(iio_device_register); **/ void iio_device_unregister(struct iio_dev *indio_dev) { - mutex_lock(&indio_dev->info_exist_lock); - device_del(&indio_dev->dev);
if (indio_dev->chrdev.dev) cdev_del(&indio_dev->chrdev); + + mutex_lock(&indio_dev->info_exist_lock); iio_device_unregister_debugfs(indio_dev);
iio_disable_all_buffers(indio_dev);