Messages in this thread | | | Date | Thu, 27 Jul 2017 07:16:33 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option |
| |
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. ;-)
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
| |