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

[Paul E. McKenney]

On Thu, Jul 13, 2023 at 11:51:34AM -0700, Sandeep Dhavale wrote:
> > Thank you for the background.
> >
> > > Paul, Joel,
> > > Shall we fix the rcu_read_lock_*held() regardless of how erofs
> > > improves the check?
> >
> > Help me out here.  Exactly what is broken with rcu_read_lock_*held(),
> > keeping in mind their intended use for lockdep-based debugging?
> >
> Hi Paul,
> With !CONFIG_DEBUG_ALLOC_LOCK
> rcu_read_lock_held() -> Always returns 1.
> rcu_read_lock_any_held()-> returns !preemptible() so may return 0.
> 
> Now current usages for rcu_read_lock_*held() are under RCU_LOCKDEP_WARN()
> which becomes noOP with !CONFIG_DEBUG_ALLOC_LOCK
> (due to debug_lockdep_rcu_enabled()) so this inconsistency is not causing
> any problems right now. So my question was about your opinion for fixing this
> for semantics if it's worth correcting.
> 
> Also it would have been better IMO if there was a reliable API
> for rcu_read_lock_*held() than erofs trying to figure it out at a higher level.

Sorry, but the current lockdep-support functions need to stay focused
on lockdep.  They are not set up for general use, as we already saw
with rcu_is_watching().

If you get a z_erofs_wq_needed() (or whatever) upstream, and if it turns
out that there is an RCU-specific portion that has clean semantics,
then I would be willing to look at pulling that portion into RCU.
Note "look at" as opposed to "unconditionally agree to".  ;-)

> > I have no official opinion myself, but there are quite a few people
> ...
> 
> Regarding erofs trying to detect this, I understand few people can
> have different
> opinions. Not scheduling a thread while being in a thread context itself
> is reasonable in my opinion which also has shown performance gains.

You still haven't quantified the performance gains.  Presumably they
are most compelling with large numbers of small buffers to be decrypted.

But why not just make a z_erofs_wq_needed() or similar in your own
code, and push it upstream?  If the performance gains really are so
compelling, one would hope that some sort of reasonable arrangement
could be arrived at.

							Thanx, Paul

> Thanks,
> Sandeep.
> 
> 
> 
> >                                                         Thanx, Paul
> >
> > > Thanks,
> > > Sandeep.
> > >
> > > [1] https://lore.kernel.org/linux-erofs/20230208093322.75816-1-hsiangkao@linux.alibaba.com/
> >
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