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SubjectRe: [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


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