Re: [PATCH rcu 13/17] srcu: Add SRCU-fast readers

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 16, 2025 at 01:00:24PM -0800, Alexei Starovoitov wrote:
> On Thu, Jan 16, 2025 at 12:21 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > +/*
> > + * Counts the new reader in the appropriate per-CPU element of the
> > + * srcu_struct.  Returns a pointer that must be passed to the matching
> > + * srcu_read_unlock_fast().
> > + *
> > + * Note that this_cpu_inc() is an RCU read-side critical section either
> > + * because it disables interrupts, because it is a single instruction,
> > + * or because it is a read-modify-write atomic operation, depending on
> > + * the whims of the architecture.
> > + */
> > +static inline struct srcu_ctr __percpu *__srcu_read_lock_fast(struct srcu_struct *ssp)
> > +{
> > +       struct srcu_ctr __percpu *scp = READ_ONCE(ssp->srcu_ctrp);
> > +
> > +       RCU_LOCKDEP_WARN(!rcu_is_watching(), "RCU must be watching srcu_read_lock_fast().");
> > +       this_cpu_inc(scp->srcu_locks.counter); /* Y */
> > +       barrier(); /* Avoid leaking the critical section. */
> > +       return scp;
> > +}
> 
> This doesn't look fast.
> If I'm reading this correctly,
> even with debugs off RCU_LOCKDEP_WARN() will still call
> rcu_is_watching() and this doesn't look cheap or fast.

this is the while (0 && (c)), thing, right? can't the compiler throw
that out in DCE ?