On 04/09, Waiman Long wrote: > > On 04/09/2018 07:20 AM, Oleg Nesterov wrote: > > Hmm. Can you look at lockdep_sb_freeze_release() and > > lockdep_sb_freeze_acquire()? > > These 2 functions are there to deal with the lockdep code.
Plus they clearly document why sem->owner check is not right when it comes to super_block->s_writers. Not only freeze and thaw can be called by different processes, we need to return to user-space with rwsem held for writing. > > At first glance, it would be much better to set sem->owner = current in > > percpu_rwsem_acquire(), no? > > The primary purpose of the owner field is to enable optimistic spinning > to improve locking performance. So it needs to be set during an > up_write() call. Unless, again, the "owner" has to do lockdep_sb_freeze_release() for any reason. And please note that percpu_rwsem_release() already clears rw_sem.owner. It checks CONFIG_RWSEM_SPIN_ON_OWNER, but this is simply because rw_semaphore->owner doesn't exist otherwise. > My rwsem debug patch does use it also to check for consistency in the > use of lock/unlock call. Anyway, I don't think it is right to set it > again in percpu_rwsem_acquire() if there is no guarantee that the task > that call percpu_rwsem_acquire will be the one that will do the unlock. Hmm. Perhaps I missed something, but I think this should be true. Of course, you need to check "if (!read)", but again, this is what percpu_rwsem_release() already does. > I am wondering if it makes sense to do optimistic spinning in the case > of percpu_rwsem where the unlocker may be a different task. Again, perhaps I missed something, but see above. percpu_rwsem does not really differ from the regular rwsem, however its usage in sb->s_writers differs. Oleg.