On Mon 14-11-22 12:44:48, Michal Hocko wrote: > On Mon 14-11-22 00:41:21, Zhongkun He wrote: > > Hi Andrew, thanks for your replay. > > > > > This sounds a bit suspicious. Please share much more detail about > > > these races. If we proced with this design then mpol_put_async() > > > shouild have comments which fully describe the need for the async free. > > > > > > How do we *know* that these races are fully prevented with this > > > approach? How do we know that mpol_put_async() won't free the data > > > until the race window has fully passed? > > > > A mempolicy can be either associated with a process or with a VMA. > > All vma manipulation is somewhat protected by a down_read on > > mmap_lock.In process context there is no locking because only > > the process accesses its own state before. > > We shouldn't really rely on mmap_sem for this IMO. There is alloc_lock > (aka task lock) that makes sure the policy is stable so that caller can > atomically take a reference and hold on the policy. And we do not do > that consistently and this should be fixed. E.g. just looking at some > random places like allowed_mems_nr (relying on get_task_policy) is > completely lockless and some paths (like fadvise) do not use any of the > explicit (alloc_lock) or implicit (mmap_lock) locking. That means that > the task_work based approach cannot really work in this case, right? Just to be more explicit. Task work based approach still requires an additional synchronization among different threads unless I miss something so this is really fragile synchronization model. -- Michal Hocko SUSE Labs
