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?
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
},
}
