linux-kernel - Re: [PATCH] rcu: Fix rcu_read_unlock

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Message-ID: <20220617001328.GD1790663@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 16 Jun 2022 17:13:28 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     frederic@...nel.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Fix rcu_read_unlock_strict() strict QS reporting

On Thu, Jun 16, 2022 at 09:53:47PM +0800, Zqiang wrote:
> When running a kerenl with CONFIG_PREEMPT=n and
> CONFIG_RCU_STRICT_GRACE_PERIOD=y, the QS state will be reported
> directly after exiting the last level of RCU critical section and
> in non irqs-disable context, but maybe the CPU's rcu_data
> structure's ->cpu_no_qs.b.norm is not cleared, as a result the
> rcu_report_qs_rdp() will exit early, and not report QS state.
> 
> This commit will clear CPU's rcu_data structure's ->cpu_no_qs.b.norm
> before invoke rcu_report_qs_rdp().
> 
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>

Good point, thank you!  Queued for review and testing with the usual
wordsmithing shown below.  As always, please check.

							Thanx, Paul

------------------------------------------------------------------------

commit cb58562318cf1fa7ad7c2c4c8d8e847c7942df66
Author: Zqiang <qiang1.zhang@...el.com>
Date:   Thu Jun 16 21:53:47 2022 +0800

    rcu: Fix rcu_read_unlock_strict() strict QS reporting
    
    Kernels built with CONFIG_PREEMPT=n and CONFIG_RCU_STRICT_GRACE_PERIOD=y
    report the quiescent state directly from the outermost rcu_read_unlock().
    However, the current CPU's rcu_data structure's ->cpu_no_qs.b.norm
    might still be set, in which case rcu_report_qs_rdp() will exit early,
    thus failing to report quiescent state.
    
    This commit therefore causes rcu_read_unlock_strict() to clear
    CPU's rcu_data structure's ->cpu_no_qs.b.norm field before invoking
    rcu_report_qs_rdp().
    
    Signed-off-by: Zqiang <qiang1.zhang@...el.com>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index b2c01919b92c0..dc78726b993fd 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -824,6 +824,7 @@ void rcu_read_unlock_strict(void)
 	if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
 		return;
 	rdp = this_cpu_ptr(&rcu_data);
+	rdp->cpu_no_qs.b.norm = false;
 	rcu_report_qs_rdp(rdp);
 	udelay(rcu_unlock_delay);
 }