Hi Thierry, On Tue, 12 Apr 2016 13:22:46 +0200 Thierry Reding <thierry.reding@xxxxxxxxx> wrote: > On Wed, Mar 30, 2016 at 10:03:27PM +0200, Boris Brezillon wrote: > > PWM devices are not protected against concurrent accesses. The lock in > > pwm_device might let PWM users think it is, but it's actually only > > protecting the enabled state. > > > > Removing this lock should be fine as long as all PWM users are aware that > > accesses to the PWM device have to be serialized, which seems to be the > > case for all of them except the sysfs interface. > > Patch the sysfs code by adding a lock to the pwm_export struct and making > > sure it's taken for all accesses to the exported PWM device. > > > > Signed-off-by: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx> > > --- > > drivers/pwm/core.c | 19 ++++-------------- > > drivers/pwm/sysfs.c | 57 ++++++++++++++++++++++++++++++++++++++++++----------- > > include/linux/pwm.h | 2 -- > > 3 files changed, 50 insertions(+), 28 deletions(-) > > This is a little overzealous. Only accesses that can cause races need to > be protected by the lock. All of the *_show() callbacks don't modify the > PWM device in any way, so there is no need to protect them against > concurrent accesses. This is probably true for this set of changes, but what will happen when we'll switch to the atomic API? There's no guarantee that pwm->state = *newstate is done atomically, and you may see a partially updated state when calling pwm_get_state() while another thread is calling pwm_apply_state(). -- Boris Brezillon, Free Electrons Embedded Linux and Kernel engineering http://free-electrons.com -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
