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

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Message-ID: <CAADnVQ+fz7DQ9O=4F-4NNo1J7=dkiDAfYa+Fc5WXBK0yH0f=TQ@mail.gmail.com>
Date: Thu, 16 Jan 2025 13:00:24 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, 
	Kernel Team <kernel-team@...a.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Kent Overstreet <kent.overstreet@...ux.dev>, 
	bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH rcu 13/17] srcu: Add SRCU-fast readers

On Thu, Jan 16, 2025 at 12:21 PM Paul E. McKenney <paulmck@...nel.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.