linux-kernel - Re: [PATCH rcu 08/15] srcu: Add srcu_read_lock_lite() and srcu_read_unlock

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Message-ID: <ZylYbsU7uE7jX5Yd@pavilion.home>
Date: Tue, 5 Nov 2024 00:27:42 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, kernel test robot <oliver.sang@...el.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Kent Overstreet <kent.overstreet@...ux.dev>, bpf@...r.kernel.org
Subject: Re: [PATCH rcu 08/15] srcu: Add srcu_read_lock_lite() and
 srcu_read_unlock_lite()

Le Tue, Oct 15, 2024 at 09:11:05AM -0700, Paul E. McKenney a écrit :
> This patch adds srcu_read_lock_lite() and srcu_read_unlock_lite(), which
> dispense with the read-side smp_mb() but also are restricted to code
> regions that RCU is watching.  If a given srcu_struct structure uses
> srcu_read_lock_lite() and srcu_read_unlock_lite(), it is not permitted
> to use any other SRCU read-side marker, before, during, or after.
> 
> Another price of light-weight readers is heavier weight grace periods.
> Such readers mean that SRCU grace periods on srcu_struct structures
> used by light-weight readers will incur at least two calls to
> synchronize_rcu().  In addition, normal SRCU grace periods for
> light-weight-reader srcu_struct structures never auto-expedite.
> Note that expedited SRCU grace periods for light-weight-reader
> srcu_struct structures still invoke synchronize_rcu(), not
> synchronize_srcu_expedited().  Something about wishing to keep
> the IPIs down to a dull roar.
> 
> The srcu_read_lock_lite() and srcu_read_unlock_lite() functions may not
> (repeat, *not*) be used from NMI handlers, but if this is needed, an
> additional flavor of SRCU reader can be added by some future commit.
> 
> [ paulmck: Apply Alexei Starovoitov expediting feedback. ]
> [ paulmck: Apply kernel test robot feedback. ]
> 
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Tested-by: kernel test robot <oliver.sang@...el.com>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Andrii Nakryiko <andrii@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Kent Overstreet <kent.overstreet@...ux.dev>
> Cc: <bpf@...r.kernel.org>

This might be a dump question but I have to ask. Could this replace
RCU-TASKS-TRACE?

Thanks.