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From:   "Zhang, Qiang" <Qiang.Zhang@windriver.com>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
CC:     "linux-rt-users@vger.kernel.org" <linux-rt-users@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Question on (locking: Make spinlock_t and rwlock_t a RCU section on
 RT)
Thread-Topic: Question on (locking: Make spinlock_t and rwlock_t a RCU section
 on RT)
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Date:   Mon, 26 Jul 2021 07:43:33 +0000
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Hello,  I have some questions about this patch =0A=
=0A=
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>=0A=
Date: Tue, 19 Nov 2019 09:25:04 +0100=0A=
Subject: [PATCH] locking: Make spinlock_t and rwlock_t a RCU section on RT=
=0A=
=0A=
On !RT a locked spinlock_t and rwlock_t disables preemption which=0A=
implies a RCU read section. There is code that relies on that behaviour.=0A=
=0A=
Add an explicit RCU read section on RT while a sleeping lock (a lock=0A=
which would disables preemption on !RT) acquired.=0A=
=0A=
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>=0A=
---=0A=
 kernel/locking/rtmutex.c   |    6 ++++++=0A=
 kernel/locking/rwlock-rt.c |    6 ++++++=0A=
 2 files changed, 12 insertions(+)=0A=
=0A=
--- a/kernel/locking/rtmutex.c=0A=
+++ b/kernel/locking/rtmutex.c=0A=
@@ -1136,6 +1136,7 @@ void __lockfunc rt_spin_lock(spinlock_t=0A=
 {=0A=
        spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);=0A=
        rt_spin_lock_fastlock(&lock->lock, rt_spin_lock_slowlock);=0A=
+       rcu_read_lock();=0A=
        migrate_disable();=0A=
...............................=0A=
=0A=
Operation of sleep is not allowed in RCU read critical area,  If  after acq=
uiring a rt-spinlock , try to acquire another one rt-spinlock,  This may ca=
use sleep, and trigger RCU warning.  =0A=
=0A=
Is there a problem with my analysis or something I missed?=0A=
=0A=
Thanks=0A=
Qiang=