Hello, Oleg. On Thu, Mar 16, 2017 at 03:54:36PM +0100, Oleg Nesterov wrote: > On 03/15, Tejun Heo wrote: > > > > Until now, all to_kthread() users are interlocked with kthread > > creation and there's no need to have explicit barriers when setting > > the kthread pointer or dereferencing it. > > > > However, There is a race condition where userland can interfere with a > > kthread while it's being initialized. To close it, to_kthread() needs > > to be used from an unsynchronized context. > > So this is preparation for 2/2... IIUC, the current code is not buggy, > just you need to add kthread_initialized() which can't work without > this change.
Yeah, I could have been clearer. > > + /* > > + * Paired with smp_wmb() in set_kthread_struct() and ensures that > > + * the caller sees initialized content of the returned kthread. > > + */ > > + smp_read_barrier_depends(); > > + > > + return ptr; > > This is almost off-topic, but I think lockless_dereference() will look > better in to_kthread(). > > And perhaps we should add another helper, say, > > #define lockless_assign_pointer(ptr, val) \ > smp_store_release(&ptr, val) > > for set_kthread_struct() ? it can have more users. > > Not that I think you should change your patch, I am just asking. Ah yeah, that would look better. I vaguely remembered the new macro but couldn't quite remember it fully. :) Will update the patch. Thanks. -- tejun
