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

[Gao Xiang]

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

Anyway, before we proceed, I also think it'd be better to get some
performance numbers first for this (e.g. with dm-verity) and record
the numbers in the commit message to justify this.  Otherwise, I guess
the same question will be raised again and again.

Thanks,
Gao Xiang