[PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC

[Joel Fernandes]

On Thu, Jul 13, 2023 at 11:17 PM Gao Xiang <hsiangkao at linux.alibaba.com> wrote:
>
>
>
> On 2023/7/14 10:16, Paul E. McKenney wrote:
> > On Thu, Jul 13, 2023 at 09:33:35AM -0700, Paul E. McKenney wrote:
> >> On Thu, Jul 13, 2023 at 11:33:24AM -0400, Joel Fernandes wrote:
>
> ...
>
> >>>
> >>> >From what Sandeep described, the code path is in an RCU reader. My
> >>> question is more, why doesn't it use SRCU instead since it clearly
> >>> does so if BLK_MQ_F_BLOCKING. What are the tradeoffs? IMHO, a deeper
> >>> dive needs to be made into that before concluding that the fix is to
> >>> use rcu_read_lock_any_held().
> >>
> >> How can this be solved?
> >>
> >> 1.   Always use a workqueue.  Simple, but is said to have performance
> >>      issues.
> >>
> >> 2.   Pass a flag in that indicates whether or not the caller is in an
> >>      RCU read-side critical section.  Conceptually simple, but might
> >>      or might not be reasonable to actually implement in the code as
> >>      it exists now.  (You tell me!)
> >>
> >> 3.   Create a function in z_erofs that gives you a decent
> >>      approximation, maybe something like the following.
> >>
> >> 4.   Other ideas here.
> >
> > 5.    #3 plus make the corresponding Kconfig option select
> >       PREEMPT_COUNT, assuming that any users needing compression in
> >       non-preemptible kernels are OK with PREEMPT_COUNT being set.
> >       (Some users of non-preemptible kernels object strenuously
> >       to the added overhead from CONFIG_PREEMPT_COUNT=y.)
>
> I'm not sure if it's a good idea

I think it is a fine idea.

> we need to work on
> CONFIG_PREEMPT_COUNT=n (why not?), we could just always trigger a
> workqueue for this.
>

So CONFIG_PREEMPT_COUNT=n users don't deserve good performance? ;-)

thanks,

 - Joel