On Wed, Jul 23, 2025 at 12:10:46PM -0400, Joel Fernandes wrote:
>
>
> On 7/23/2025 9:32 AM, joelagn...@nvidia.com wrote:
> >
> >
> >> On Jul 22, 2025, at 6:17 PM, Paul E. McKenney <paul...@kernel.org> wrote:
> >>
> >> This commit documents the implicit RCU readers that are implied by the
> >> this_cpu_inc() and atomic_long_inc() operations in __srcu_read_lock_fast()
> >> and __srcu_read_unlock_fast(). While in the area, fix the documentation
> >> of the memory pairing of atomic_long_inc() in __srcu_read_lock_fast().
> >
> > Just to clarify, the implication here is since SRCU-fast uses
> > synchronize_rcu on the update side, these operations result in blocking of
> > classical RCU too. So simply using srcu fast is another way of achieving
> > the previously used pre-empt-disabling in the use cases.
>
> Hi Paul, it was nice sync'ing with you off-list. Following are my suggestions
> and where I am coming from:
>
> 1. For someone who doesn't know SRCU-fast depends on synchronize_rcu (me
> after a
> few beers :P), the word 'RCU' in the comment you added to this patch, might
> come
> across as 'which RCU are we referring to - SRCU or classical RCU or some
> other'.
> So I would call it 'classical RCU reader' in the comment.
>
> 2. It would be good to call out specifically that, the SRCU-fast critical
> section is akin to a classical RCU reader, because of its implementation's
> dependence on synchronize_rcu() to overcome the lack of read-side memory
> barriers.
>
> 3. I think since the potential size of these code comment suggestions, it may
> make sense to provide a bigger comment suggesting these than providing them
> inline as you did. And also calling out the tracing usecase in the comments
> for
> additional usecase clarification.
>
> I could provide a patch to do all this soon, as we discussed, as well (unless
> you're Ok with making this change as well).
Thank you very much for the clarification, and I will make the changes
with attribution.
Thanx, Paul
> Thanks!
>
> - Joel
>
>
>
>
> >
> > Or is the rationale for this something else?
> >
> > I would probably spell this out more in a longer comment above the if/else,
> > than modify the inline comments.
> >
> > But I am probably misunderstood the whole thing. :-(
> >
> > -Joel
> >
> >>
> >> Signed-off-by: Paul E. McKenney <paul...@kernel.org>
> >> Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
> >> Cc: Steven Rostedt <rost...@goodmis.org>
> >> Cc: Sebastian Andrzej Siewior <bige...@linutronix.de>
> >> Cc: <b...@vger.kernel.org>
> >>
> >> diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> >> index 043b5a67ef71e..78e1a7b845ba9 100644
> >> --- a/include/linux/srcutree.h
> >> +++ b/include/linux/srcutree.h
> >> @@ -245,9 +245,9 @@ static inline struct srcu_ctr __percpu
> >> *__srcu_read_lock_fast(struct srcu_struct
> >> struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp);
> >>
> >> if (!IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE))
> >> - this_cpu_inc(scp->srcu_locks.counter); /* Y */
> >> + this_cpu_inc(scp->srcu_locks.counter); // Y, and implicit RCU
> >> reader.
> >> else
> >> - atomic_long_inc(raw_cpu_ptr(&scp->srcu_locks)); /* Z */
> >> + atomic_long_inc(raw_cpu_ptr(&scp->srcu_locks)); // Y, and
> >> implicit RCU reader.
> >> barrier(); /* Avoid leaking the critical section. */
> >> return scp;
> >> }
> >> @@ -271,9 +271,9 @@ static inline void __srcu_read_unlock_fast(struct
> >> srcu_struct *ssp, struct srcu_
> >> {
> >> barrier(); /* Avoid leaking the critical section. */
> >> if (!IS_ENABLED(CONFIG_NEED_SRCU_NMI_SAFE))
> >> - this_cpu_inc(scp->srcu_unlocks.counter); /* Z */
> >> + this_cpu_inc(scp->srcu_unlocks.counter); // Z, and implicit RCU
> >> reader.
> >> else
> >> - atomic_long_inc(raw_cpu_ptr(&scp->srcu_unlocks)); /* Z */
> >> + atomic_long_inc(raw_cpu_ptr(&scp->srcu_unlocks)); // Z, and
> >> implicit RCU reader.
> >> }
> >>
> >> void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor);
> >>
>
