On 08/18/2017 12:21 PM, Bart Van Assche wrote: > On Fri, 2017-08-18 at 09:55 -0400, Waiman Long wrote: >> On 08/17/2017 05:30 PM, Steven Rostedt wrote: >>> On Thu, 17 Aug 2017 17:10:07 -0400 >>> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: >>>> Instead of playing games with taking the lock, the only way this race >>>> is hit, is if the partition is being deleted and the sysfs attribute is >>>> being read at the same time, correct? In that case, just return >>>> -ENODEV, and be done with it. >>> Nevermind that wont work. Too bad there's not a mutex_lock_timeout() >>> that we could use in a loop. It would solve the issue of forward >>> progress with RT tasks, and will break after a timeout in case of >>> deadlock. >> I think it will be useful to have mutex_timed_lock(). RT-mutex does have >> a timed version, so I guess it shouldn't be hard to implement one for >> mutex. I can take a shot at trying to do that. > (just caught up with the entire e-mail thread) > > Sorry Waiman but personally I thoroughly detest loops around mutex_trylock() or > mutex_timed_lock() because such loops are usually used to paper over a problem > instead of fixing the root cause. What I understood from the comment in v1 of your > patch is that bd_mutex is not only held during block device creation and removal > but additionally that bd_mutex is obtained inside sysfs attribute callback methods? > That pattern is guaranteed to lead to deadlocks. Since the block device removal > code waits until all sysfs callback methods have finished there is no need to > protect against block device removal inside the sysfs callback methods. My proposal You are right. We don't really need to take the bd_mutex as the fact that inside the sysfs callback method will guarantee the block device won't go away. > is to split bd_mutex: one global mutex that serializes block device creation and > removal and one mutex per block device that serializes changes to a single block > device. Obtaining the global mutex from inside a block device sysfs callback > function is not safe but obtaining the per-block-device mutex from inside a sysfs > callback function is safe. > > Bart. The bd_mutex we are talking here is already per block device. I am thinking about having a global blktrace mutex that is used to serialize the read and write of blktrace attributes. Since blktrace sysfs files are not supposed to be frequently accessed, having a global lock shouldn't cause any problem. Thanks, Longman
