Messages in this thread | | | Date | Thu, 27 Jul 2017 07:55:32 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option |
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On Thu, Jul 27, 2017 at 10:47:03PM +0800, Boqun Feng wrote: > On Thu, Jul 27, 2017 at 07:36:58AM -0700, Paul E. McKenney wrote: > > > > > > > > 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. > > > > All those barrier guarantees are subject to a RCU read-side critical > section with a synchonize_*(), IIRC, for example: > > * On systems with more than one CPU, when synchronize_sched() returns, > * each CPU is guaranteed to have executed a full memory barrier since the > * end of its last RCU-sched read-side critical section whose beginning > * preceded the call to synchronize_sched(). In addition, each CPU having > > , which is not the case for a quiesent state without a read-side > critical section(i.e. non-context-switch quiesent state for sched Flavor) > > I've read those requirements and could not find one to explain why there > will be a full barrier emitted in an interrupted user-space program.
What you are forgetting is that for synchronize_sched(), any region of code with preemption disabled is an RCU-sched read-side critical section.
Thanx, Paul
> Regards, > Boqun > > > 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 > > > > > > > > > > > >
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