On Thu, Jan 16, 2025 at 12:21 PM Paul E. McKenney <[email protected]> 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.
