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

[Gao Xiang]

On 2023/7/13 10:02, 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:
>
> 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

Sorry about the incorrect link (I really don't know who initally reported
this and on which device):

https://lore.kernel.org/linux-erofs/161f1615-3d85-cf47-d2d5-695adf1ca7d4@xxxxxxxxxxxxxxxxx/T/#t

>   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.
>
> Hopefully I show it more clear.
>
> Thanks,
> Gao Xiang