From: "Paul E. McKenney" <paulmck@kernel.org>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org,
Frederic Weisbecker <frederic@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Marco Elver <elver@google.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
"Uladzislau Rezki (Sony)" <urezki@gmail.com>,
fweisbec@gmail.com, neeraj.iitr10@gmail.com
Subject: Re: [PATCH v9 6/7] rcu/tree: segcblist: Remove redundant smp_mb()s
Date: Wed, 4 Nov 2020 19:57:13 -0800 [thread overview]
Message-ID: <20201105035713.GL3249@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20201103142603.1302207-7-joel@joelfernandes.org>
On Tue, Nov 03, 2020 at 09:26:02AM -0500, Joel Fernandes (Google) wrote:
> This memory barrier is not needed as rcu_segcblist_add_len() already
> includes a memory barrier *before* and *after* the length of the list is
> updated.
>
> Same reasoning for rcu_segcblist_enqueue().
I suggest a commit log like the following:
------------------------------------------------------------------------
The full memory barriers in rcu_segcblist_enqueue() and in rcu_do_batch()
are not needed because rcu_segcblist_add_len(), and thus also
rcu_segcblist_inc_len(), already includes a memory barrier *before*
and *after* the length of the list is updated.
This commit therefore removes these redundant smp_mb() invocations.
------------------------------------------------------------------------
Other than that, looks good! I could hand-apply it, but that
would just cause more churn with the addition of the call to
rcu_segcblist_inc_seglen(). So could you please update the commit log
when you repost, whether to the mailing list or from your git tree?
Thanx, Paul
> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> kernel/rcu/rcu_segcblist.c | 1 -
> kernel/rcu/tree.c | 1 -
> 2 files changed, 2 deletions(-)
>
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index e9e72d72f7a6..d96272e8d604 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -268,7 +268,6 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
> struct rcu_head *rhp)
> {
> rcu_segcblist_inc_len(rsclp);
> - smp_mb(); /* Ensure counts are updated before callback is enqueued. */
> rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
> rhp->next = NULL;
> WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index f6c6653b3ec2..fb2a5ac4a59c 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2525,7 +2525,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
>
> /* Update counts and requeue any remaining callbacks. */
> rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);
> - smp_mb(); /* List handling before counting for rcu_barrier(). */
> rcu_segcblist_add_len(&rdp->cblist, -count);
>
> /* Reinstate batch limit if we have worked down the excess. */
> --
> 2.29.1.341.ge80a0c044ae-goog
>
next prev parent reply other threads:[~2020-11-05 3:57 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-03 14:25 [PATCH v9 0/7] Add support for length of each segment in the segcblist Joel Fernandes (Google)
2020-11-03 14:25 ` [PATCH v9 1/7] rcu/tree: Make rcu_do_batch count how many callbacks were executed Joel Fernandes (Google)
2020-11-04 0:05 ` Paul E. McKenney
2020-11-04 15:09 ` Joel Fernandes
2020-11-03 14:25 ` [PATCH v9 2/7] rcu/segcblist: Add counters to segcblist datastructure Joel Fernandes (Google)
2020-11-04 13:37 ` Neeraj Upadhyay
2020-11-04 17:01 ` Paul E. McKenney
2020-11-07 0:01 ` Joel Fernandes
2020-11-07 0:18 ` Joel Fernandes
2020-11-07 0:48 ` Paul E. McKenney
2020-11-10 1:39 ` Paul E. McKenney
2020-11-03 14:25 ` [PATCH v9 3/7] srcu: Fix invoke_rcu_callbacks() segcb length adjustment Joel Fernandes (Google)
2020-11-03 14:47 ` Frederic Weisbecker
2020-11-03 14:56 ` Joel Fernandes
2020-11-03 15:07 ` Joel Fernandes
2020-11-03 15:18 ` Paul E. McKenney
2020-11-03 15:19 ` Frederic Weisbecker
2020-11-04 13:37 ` Neeraj Upadhyay
2020-11-03 14:26 ` [PATCH v9 4/7] rcu/trace: Add tracing for how segcb list changes Joel Fernandes (Google)
2020-11-03 15:17 ` Frederic Weisbecker
2020-11-04 14:08 ` Paul E. McKenney
2020-11-04 14:33 ` Frederic Weisbecker
2020-11-07 0:05 ` Joel Fernandes
2020-11-04 13:40 ` Neeraj Upadhyay
2020-11-04 15:05 ` Joel Fernandes
2020-11-11 0:35 ` Paul E. McKenney
2020-11-11 13:49 ` Steven Rostedt
2020-11-11 14:08 ` Joel Fernandes
2020-11-03 14:26 ` [PATCH v9 5/7] rcu/segcblist: Remove useless rcupdate.h include Joel Fernandes (Google)
2020-11-05 3:48 ` Paul E. McKenney
2020-11-05 14:28 ` Paul E. McKenney
2020-11-07 0:27 ` Joel Fernandes
2020-11-03 14:26 ` [PATCH v9 6/7] rcu/tree: segcblist: Remove redundant smp_mb()s Joel Fernandes (Google)
2020-11-05 3:57 ` Paul E. McKenney [this message]
2020-11-07 0:26 ` Joel Fernandes
2020-11-10 1:41 ` Paul E. McKenney
2020-11-03 14:26 ` [PATCH v9 7/7] rcu/segcblist: Add additional comments to explain smp_mb() Joel Fernandes (Google)
2020-11-05 18:55 ` Paul E. McKenney
2020-11-06 22:41 ` Joel Fernandes
2020-11-10 1:28 ` Paul E. McKenney
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