Message ID | 20230908203603.5865-5-frederic@kernel.org |
---|---|
State | New |
Headers |
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[2620:137:e000::1:20]) by mx.google.com with ESMTP id u2-20020a17090282c200b001b67bdc438csi2137358plz.376.2023.09.08.14.50.57; Fri, 08 Sep 2023 14:51:04 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20; Authentication-Results: mx.google.com; dkim=pass header.i=@kernel.org header.s=k20201202 header.b=cCmajFEM; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344101AbjIHUgb (ORCPT <rfc822;realc9580@gmail.com> + 39 others); Fri, 8 Sep 2023 16:36:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52250 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1344135AbjIHUg1 (ORCPT <rfc822;linux-kernel@vger.kernel.org>); Fri, 8 Sep 2023 16:36:27 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 598E3C4; Fri, 8 Sep 2023 13:36:23 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 18509C433B9; Fri, 8 Sep 2023 20:36:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1694205383; bh=GUT92MKTD1zv1ELbTZztdEp1nZcVqJTBRlzdjoDETLs=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=cCmajFEMvoh8fTOLErYAwt+nF4hfVHNs6QQNNM/2Y8GtU1lnueDle9kgxUkMdxyby DOv1MXqFeu+ZgrAlsLMXbEGswqAyH9ONebKnlQ7L48/FEeGD5EctI+M7bZsHCWRPXy RRuE5dO1nT0G/fxa4WuI2+XtD1nMAWDglSuzgODLlxHwQ7p6eCQA1J9WQ1pZfz4ZUq vh1loZHLvNHYNxGyO8d1W4S7G0Z+GKq21flczenkRJZyt0obo0vypJ2mL6wtMRAYBx /sSjXr+DxA0M9ffpOqnb70Tuj01OKYMRPqqJw8j1dACnMf+MIF4fhnVttQVqFemWqP X+Y93EB2Ad+Eg== From: Frederic Weisbecker <frederic@kernel.org> To: "Paul E . McKenney" <paulmck@kernel.org> Cc: LKML <linux-kernel@vger.kernel.org>, Frederic Weisbecker <frederic@kernel.org>, rcu <rcu@vger.kernel.org>, Uladzislau Rezki <urezki@gmail.com>, Neeraj Upadhyay <quic_neeraju@quicinc.com>, Boqun Feng <boqun.feng@gmail.com>, Joel Fernandes <joel@joelfernandes.org> Subject: [PATCH 04/10] rcu/nocb: Remove needless full barrier after callback advancing Date: Fri, 8 Sep 2023 22:35:57 +0200 Message-ID: <20230908203603.5865-5-frederic@kernel.org> X-Mailer: git-send-email 2.41.0 In-Reply-To: <20230908203603.5865-1-frederic@kernel.org> References: <20230908203603.5865-1-frederic@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: <linux-kernel.vger.kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1776507803041482964 X-GMAIL-MSGID: 1776507803041482964 |
Series |
rcu cleanups
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Commit Message
Frederic Weisbecker
Sept. 8, 2023, 8:35 p.m. UTC
A full barrier is issued from nocb_gp_wait() upon callbacks advancing
to order grace period completion with callbacks execution.
However these two events are already ordered by the
smp_mb__after_unlock_lock() barrier within the call to
raw_spin_lock_rcu_node() that is necessary for callbacks advancing to
happen.
The following litmus test shows the kind of guarantee that this barrier
provides:
C smp_mb__after_unlock_lock
{}
// rcu_gp_cleanup()
P0(spinlock_t *rnp_lock, int *gpnum)
{
// Grace period cleanup increase gp sequence number
spin_lock(rnp_lock);
WRITE_ONCE(*gpnum, 1);
spin_unlock(rnp_lock);
}
// nocb_gp_wait()
P1(spinlock_t *rnp_lock, spinlock_t *nocb_lock, int *gpnum, int *cb_ready)
{
int r1;
// Call rcu_advance_cbs() from nocb_gp_wait()
spin_lock(nocb_lock);
spin_lock(rnp_lock);
smp_mb__after_unlock_lock();
r1 = READ_ONCE(*gpnum);
WRITE_ONCE(*cb_ready, 1);
spin_unlock(rnp_lock);
spin_unlock(nocb_lock);
}
// nocb_cb_wait()
P2(spinlock_t *nocb_lock, int *cb_ready, int *cb_executed)
{
int r2;
// rcu_do_batch() -> rcu_segcblist_extract_done_cbs()
spin_lock(nocb_lock);
r2 = READ_ONCE(*cb_ready);
spin_unlock(nocb_lock);
// Actual callback execution
WRITE_ONCE(*cb_executed, 1);
}
P3(int *cb_executed, int *gpnum)
{
int r3;
WRITE_ONCE(*cb_executed, 2);
smp_mb();
r3 = READ_ONCE(*gpnum);
}
exists (1:r1=1 /\ 2:r2=1 /\ cb_executed=2 /\ 3:r3=0) (* Bad outcome. *)
Here the bad outcome only occurs if the smp_mb__after_unlock_lock() is
removed. This barrier orders the grace period completion against
callbacks advancing and even later callbacks invocation, thanks to the
opportunistic propagation via the ->nocb_lock to nocb_cb_wait().
Therefore the smp_mb() placed after callbacks advancing can be safely
removed.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/rcu/tree_nocb.h | 1 -
1 file changed, 1 deletion(-)
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 6e63ba4788e1..2dc76f5e6e78 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -779,7 +779,6 @@ static void nocb_gp_wait(struct rcu_data *my_rdp) if (rcu_segcblist_ready_cbs(&rdp->cblist)) { needwake = rdp->nocb_cb_sleep; WRITE_ONCE(rdp->nocb_cb_sleep, false); - smp_mb(); /* CB invocation -after- GP end. */ } else { needwake = false; }