On Mon, Nov 23, 2020 at 01:59:37AM +0000, Zhang, Qiang wrote:
>
>
> ________________________________________
> 发件人: Paul E. McKenney <paulmck@xxxxxxxxxx>
> 发送时间: 2020年11月21日 9:11
> 收件人: Zhang, Qiang
> 抄送: rcu@xxxxxxxxxxxxxxx; Joel Fernandes
> 主题: Re: SRCU: Question on srcu_advance_state
>
> [Please note this e-mail is from an EXTERNAL e-mail address]
>
> On Fri, Nov 20, 2020 at 04:30:15PM +0800, Zhang,Qiang wrote:
> > Hello Pual
> >
> > sorry to disturb you, I also have some quesetion for you:
> >
> > in srcu_advance_state function, when seq state == SRCU_STATE_SCAN1, we will
> > check, if the previous readers critical region exits, will be returned
> > directly.
> >
> > I'd like to know under what scenario this will happen?
>
> >Suppose that a reader gets preempted for a very long time in the
> >middle of __srcu_read_lock():
> >
> > int __srcu_read_lock(struct srcu_struct *ssp)
> > {
> > int idx;
> >
> > idx = READ_ONCE(ssp->srcu_idx) & 0x1;
> > // PREEMPTED RIGHT HERE
> > this_cpu_inc(ssp->sda->srcu_lock_count[idx]);
> > smp_mb(); /* B */ /* Avoid leaking the critical section. */
> > return idx;
> > }
>
> >Suppose that several SRCU grace periods elapse during that preemption.
> >
> >Do you see how that can lead to your SRCU_STATE_SCAN1 early exit?
>
> I see, thank you for your explanation.
To cement your new knowledge, can you tell me in detail the sequence of
events, line by line in the code, that would lead to the SRCU_STATE_SCAN1
early exit?
Thanx, Paul
> Thanks
> Qiang
> >
> > Thanx, Paul
> >
> >
> >PS: The paper below outlines a similar situation in userspace RCU,
> > so feel free to use this paper as a hint. The paper is in two
> > pieces, with the first piece in the first URL and the second in
> > either of the last two URLs.
> >
> >@article{MathieuDesnoyers2012URCU,
> > Author="Mathieu Desnoyers and Paul E. McKenney and Alan Stern and Michel >R. Dagenais and Jonathan Walpole",
> > Title="User-Level Implementations of Read-Copy Update",
> >journal="IEEE Transactions on Parallel and Distributed Systems",
> >volume={23},
> > year="2012",
> >issn="1045-9219",
> > pages="375-382",
> > doi="10.1109/TPDS.2011.159",
> >publisher="IEEE Computer Society",
> > address="Los Alamitos, CA, USA",
> > annotation={
> > RCU overview, desiderata, semi-formal semantics, user-level RCU
> > usage scenarios, three classes of RCU implementation, wait-free
> > RCU updates, RCU grace-period batching, update overhead,
> > http://www.rdrop.com/users/paulmck/RCU/urcu-main-accepted.2011.08.30a.pdf
> > http://www.rdrop.com/users/paulmck/RCU/urcu-supp-accepted.2011.08.30a.pdf
> > http://www.computer.org/cms/Computer.org/dl/trans/td/2012/02/extras/ttd2012020375s.pdf
> >},
> >}
