On Thu, Jul 27, 2017 at 10:29:55PM +0800, Boqun Feng wrote: > On Thu, Jul 27, 2017 at 07:16:33AM -0700, Paul E. McKenney wrote: > > On Thu, Jul 27, 2017 at 09:55:51PM +0800, Boqun Feng wrote: > > > Hi Paul, > > > > > > I have a side question out of curiosity: > > > > > > How does synchronize_sched() work properly for sys_membarrier()? > > > > > > sys_membarrier() requires every other CPU does a smp_mb() before it > > > returns, and I know synchronize_sched() will wait until all CPUs running > > > a kernel thread do a context-switch, which has a smp_mb(). However, I > > > believe sched flavor RCU treat CPU running a user thread as a quiesent > > > state, so synchronize_sched() could return without that CPU does a > > > context switch. > > > > > > So why could we use synchronize_sched() for sys_membarrier()? > > > > > > In particular, could the following happens? > > > > > > CPU 0: CPU 1: > > > ========================= ========================== > > > <in user space> <in user space> > > > {read Y}(reordered) > > > <------------------------------+ > > > store Y; > > > | > > > read X; > > > --------------------------------------+ | > > > sys_membarrier(): <timer interrupt> > > > | | > > > synchronize_sched(); update_process_times(user): //user == > > > true | | > > > rcu_check_callbacks(usr): > > > | | > > > if (user || ..) { > > > | | > > > rcu_sched_qs() > > > | | > > > ... > > > | | > > > <report quesient state in > > > softirq> | | > > > > The reporting of the quiescent state will acquire the leaf rcu_node > > structure's lock, with an smp_mb__after_unlock_lock(), which will > > one way or another be a full memory barrier. So the reorderings > > cannot happen. > > > > Unless I am missing something subtle. ;-) > > > > Well, smp_mb__after_unlock_lock() in ARM64 is a no-op, and ARM64's lock > doesn't provide a smp_mb(). > > So my point is more like: synchronize_sched() happens to be a > sys_membarrier() because of some implementation detail, and if some day > we come up with a much cheaper way to implement sched flavor > RCU(hopefully!), synchronize_sched() may be not good for the job. So at > least, we'd better document this somewhere?
Last I heard, ARM's unlock/lock acted as a full barrier. Will? Please see the synchronize_sched() comment header for the documentation you are asking for. And the "Memory-Barrier Guarantees" section of Documentation/RCU/Design/Requirements/Requirements.html. Thanx, Paul > Regards, > Boqun > > > Thanx, Paul > > > > > <return to user space> > > > | | > > > read Y; > > > --------------------------------------+----+ > > > store X; > > > | > > > {read X}(reordered) > > > <-------------------------+ > > > > > > I assume the timer interrupt handler, which interrupts a user space and > > > reports a quiesent state for sched flavor RCU, may not have a smp_mb() > > > in some code path. > > > > > > I may miss something subtle, but it just not very obvious how > > > synchronize_sched() will guarantee a remote CPU running in userspace to > > > do a smp_mb() before it returns, this is at least not in RCU > > > requirements, right? > > > > > > Regards, > > > Boqun > > > >