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?
>>>
Suppose you have the following scenario :
ssp->srcu_idx = 0
cpu0 cpu1
__srcu_read_lock process_srcu
idx = READ_ONCE(ssp->srcu_idx) & 0x1; srcu_advance_state
/*idx == 0,*/ by preempt long time SRCU_STATE_SCAN1
idx = 1 ^ (ssp->srcu_idx & 1);
/*idx == 1*/
try_check_zero (idx)== true
other task run srcu_flip(ssp);
/* ssp->srcu_idx == 1*/
SRCU_STATE_SCAN2
idx = 1 ^ (ssp->srcu_idx & 1);
/*idx == 0*/
try_check_zero (idx)== true
srcu_gp_end
current task run
this_cpu_inc(ssp->sda->srcu_lock_count[idx]); process_srcu
srcu_advance_state
SRCU_STATE_SCAN1
/*ssp->srcu_idx ==1*/
idx = 1 ^ (ssp->srcu_idx & 1);
/*idx==0*/
try_check_zero == false
return
do something
__srcu_read_unlock
/*idx == 0*/
this_cpu_inc(ssp->sda->srcu_unlock_count[idx]);
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
> >},
> >}
