On Tue 15-05-18 10:35:25, Peter Zijlstra wrote: > On Mon, May 14, 2018 at 03:31:07PM -0400, Waiman Long wrote: > > The percpu_rwsem_release() is called when the ownership of the embedded > > rwsem is to be transferred to another task. The new owner, however, may > > take a while to get the ownership of the lock via percpu_rwsem_acquire(). > > During that period, the rwsem is now marked as writer-owned with no > > optimistic spinning. > > This does not explain the problem sufficiently to even begin considering > if the proposed solution is sensible. So the original problem is following: There is percpu_rw_semaphore in super_block which is used to implement filesystem freezing (actually three of them but that's not really substantial here). This semaphore is acquired for writing when a fs is frozen (i.e., in response to a syscall) and we return to userspace with this semaphore held. Later someone else calls another syscall to unfreeze the filesystem which drops the semaphore. Now this behavior upsets lockdep and that's why we fool it by telling the semaphore got released before returning to userspace (through percpu_rwsem_release() helper) and similarly we tell lockdep we've got the semaphore when an unfreeze syscall is called by percpu_rwsem_acquire(). Now Amir has discovered that also rwsem debugging code gets confused by this behavior and previously also someone noticed that rwsem spinning does not make sense and can be broken by this behavior. So these patches from Waiman try to fix up all these problems... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
