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

[Gao Xiang]

On 2023/7/13 12:27, Paul E. McKenney wrote:
> On Thu, Jul 13, 2023 at 10:02:17AM +0800, Gao Xiang wrote:
> > On 2023/7/13 08:32, Joel Fernandes wrote:
> > > On Wed, Jul 12, 2023 at 02:20:56PM -0700, Sandeep Dhavale wrote:
> > > [..]
> > > > > As such this patch looks correct to me, one thing I noticed is that
> > > > > you can check rcu_is_watching() like the lockdep-enabled code does.
> > > > > That will tell you also if a reader-section is possible because in
> > > > > extended-quiescent-states, RCU readers should be non-existent or
> > > > > that's a bug.
> > > > Please correct me if I am wrong, reading from the comment in
> > > > kernel/rcu/update.c rcu_read_lock_held_common()
> > > > ..
> > > >     * The reason for this is that RCU ignores CPUs that are
> > > >    * in such a section, considering these as in extended quiescent state,
> > > >    * so such a CPU is effectively never in an RCU read-side critical section
> > > >    * regardless of what RCU primitives it invokes.
> > > >
> > > > It seems rcu will treat this as lock not held rather than a fact that
> > > > lock is not held. Is my understanding correct?
> > >
> > > If RCU treats it as a lock not held, that is a fact for RCU ;-). Maybe you
> > > mean it is not a fact for erofs?
> >
> > I'm not sure if I get what you mean, EROFS doesn't take any RCU read lock
> > here:
>
> The key point is that we need lockdep to report errors when
> rcu_read_lock(), rcu_dereference(), and friends are used when RCU is
> not watching.  We also need lockdep to report an error when someone
> uses rcu_dereference() when RCU is not watching, but also forgets the
> rcu_read_lock().
>
> And this is the job of rcu_read_lock_held(), which is one reason why
> that rcu_is_watching() is needed.
>
> > z_erofs_decompressqueue_endio() is actually a "bio->bi_end_io", previously
> > which can be called under two scenarios:
> >
> >   1) under softirq context, which is actually part of device I/O compleltion;
> >
> >   2) under threaded context, like what dm-verity or likewise calls.
> >
> > But EROFS needs to decompress in a threaded context anyway, so we trigger
> > a workqueue to resolve the case 1).
> >
> > Recently, someone reported there could be some case 3) [I think it was
> > introduced recently but I have no time to dig into it]:
> >
> >   case 3: under RCU read lock context, which is shown by this:
> > https://lore.kernel.org/r/4a8254eb-ac39-1e19-3d82-417d3a7b9f94@xxxxxxxxxxxxxxxxx/T/#u
> >
> >   and such RCU read lock is taken in __blk_mq_run_dispatch_ops().
> >
> > But as the commit shown, we only need to trigger a workqueue for case 1)
> > and 3) due to performance reasons.
>
> Just out of curiosity, exactly how much is it costing to trigger the
> workqueue?

There are lots of performance issues here and even a plumber
topic last year to show that, see:

[1] https://lore.kernel.org/r/20230519001709.2563-1-tj@xxxxxxxxxx
[2] https://lore.kernel.org/r/CAHk-=wgE9kORADrDJ4nEsHHLirqPCZ1tGaEPAZejHdZ03qCOGg@xxxxxxxxxxxxxx
[3] https://lore.kernel.org/r/CAB=BE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8GbO6wyUqFneQ@xxxxxxxxxxxxxx
[4] https://lpc.events/event/16/contributions/1338/
and more.

I'm not sure if it's necessary to look info all of that,
andSandeep knows more than I am (the scheduling issue
becomes vital on some aarch64 platform.)

Thanks,
Gao Xiang