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Date:   Sun, 12 Jun 2022 11:22:09 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Zqiang <qiang1.zhang@intel.com>
Cc:     joel@joelfernandes.org, frederic@kernel.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] refscale: Convert test_lock spinlock to raw_spinlock
Message-ID: <20220612182209.GT1790663@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20220612020225.3098239-1-qiang1.zhang@intel.com>
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On Sun, Jun 12, 2022 at 10:02:25AM +0800, Zqiang wrote:
> In PREEMPT_RT kernel, the spinlock are replaced by sleepable rt_mutex.
> in a critical section where preemption is disabled this will trigger
> calltrace like the following:
> 
> BUG: scheduling while atomic: ref_scale_reade/76/0x00000002
> Preemption disabled at:
> ref_lock_section+0x16/0x80
> Call Trace:
> <TASK>
> dump_stack_lvl+0x5b/0x82
> dump_stack+0x10/0x12
> __schedule_bug.cold+0x9c/0xad
> __schedule+0x839/0xc00
> schedule_rtlock+0x22/0x40
> rtlock_slowlock_locked+0x460/0x1350
> rt_spin_lock+0x61/0xe0
> ref_lock_section+0x29/0x80
> rcu_scale_one_reader+0x52/0x60
> ref_scale_reader+0x28d/0x490
> kthread+0x128/0x150
> ret_from_fork+0x22/0x30
> </TASK>
> 
> Convert spinlock to raw_spinlock to fix it.
> 
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>

Queued for review and testing, thank you!

And as usual, wordsmithed, so could you please check for anything
that I might have messed up?

							Thanx, Paul

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

commit 0c284371063996f09c0b5e3d0f4c7dff8e966d86
Author: Zqiang <qiang1.zhang@intel.com>
Date:   Sun Jun 12 10:02:25 2022 +0800

    refscale: Convert test_lock spinlock to raw_spinlock
    
    In kernels built with CONFIG_PREEMPT_RT=y, spinlocks are replaced by
    rt_mutex, which can sleep.  This means that acquiring a non-raw spinlock
    in a critical section where preemption is disabled can trigger the
    following BUG:
    
    BUG: scheduling while atomic: ref_scale_reade/76/0x00000002
    Preemption disabled at:
    ref_lock_section+0x16/0x80
    Call Trace:
    <TASK>
    dump_stack_lvl+0x5b/0x82
    dump_stack+0x10/0x12
    __schedule_bug.cold+0x9c/0xad
    __schedule+0x839/0xc00
    schedule_rtlock+0x22/0x40
    rtlock_slowlock_locked+0x460/0x1350
    rt_spin_lock+0x61/0xe0
    ref_lock_section+0x29/0x80
    rcu_scale_one_reader+0x52/0x60
    ref_scale_reader+0x28d/0x490
    kthread+0x128/0x150
    ret_from_fork+0x22/0x30
    </TASK>
    
    This commit therefore converts spinlock to raw_spinlock.
    
    Signed-off-by: Zqiang <qiang1.zhang@intel.com>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
index 909644abee67f..435c884c02b5c 100644
--- a/kernel/rcu/refscale.c
+++ b/kernel/rcu/refscale.c
@@ -385,7 +385,7 @@ static struct ref_scale_ops rwsem_ops = {
 };
 
 // Definitions for global spinlock
-static DEFINE_SPINLOCK(test_lock);
+static DEFINE_RAW_SPINLOCK(test_lock);
 
 static void ref_lock_section(const int nloops)
 {
@@ -393,8 +393,8 @@ static void ref_lock_section(const int nloops)
 
 	preempt_disable();
 	for (i = nloops; i >= 0; i--) {
-		spin_lock(&test_lock);
-		spin_unlock(&test_lock);
+		raw_spin_lock(&test_lock);
+		raw_spin_unlock(&test_lock);
 	}
 	preempt_enable();
 }
@@ -405,9 +405,9 @@ static void ref_lock_delay_section(const int nloops, const int udl, const int nd
 
 	preempt_disable();
 	for (i = nloops; i >= 0; i--) {
-		spin_lock(&test_lock);
+		raw_spin_lock(&test_lock);
 		un_delay(udl, ndl);
-		spin_unlock(&test_lock);
+		raw_spin_unlock(&test_lock);
 	}
 	preempt_enable();
 }
@@ -427,8 +427,8 @@ static void ref_lock_irq_section(const int nloops)
 
 	preempt_disable();
 	for (i = nloops; i >= 0; i--) {
-		spin_lock_irqsave(&test_lock, flags);
-		spin_unlock_irqrestore(&test_lock, flags);
+		raw_spin_lock_irqsave(&test_lock, flags);
+		raw_spin_unlock_irqrestore(&test_lock, flags);
 	}
 	preempt_enable();
 }
@@ -440,9 +440,9 @@ static void ref_lock_irq_delay_section(const int nloops, const int udl, const in
 
 	preempt_disable();
 	for (i = nloops; i >= 0; i--) {
-		spin_lock_irqsave(&test_lock, flags);
+		raw_spin_lock_irqsave(&test_lock, flags);
 		un_delay(udl, ndl);
-		spin_unlock_irqrestore(&test_lock, flags);
+		raw_spin_unlock_irqrestore(&test_lock, flags);
 	}
 	preempt_enable();
 }