linux-kernel - Re: [PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi

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Date:   Thu, 21 Apr 2022 11:36:17 +0800
From:   Pingfan Liu <kernelfans@...il.com>
To:     Heiko Carstens <hca@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse()
 in show_msi_interrupt()

On Wed, Apr 20, 2022 at 08:16:37PM +0200, Heiko Carstens wrote:
> On Wed, Apr 20, 2022 at 10:05:16PM +0800, Pingfan Liu wrote:
> > irq_desc can be accessed safely in RCU read section as demonstrated by
> > kstat_irqs_usr(). And raw_spin_lock_irqsave() context can provide a rcu
> > read section, which can be utilized to get rid of irq_lock_sparse().
> > 
> > Signed-off-by: Pingfan Liu <kernelfans@...il.com>
> > Cc: Heiko Carstens <hca@...ux.ibm.com>
> > Cc: Vasily Gorbik <gor@...ux.ibm.com>
> > Cc: Alexander Gordeev <agordeev@...ux.ibm.com>
> > Cc: Christian Borntraeger <borntraeger@...ux.ibm.com>
> > Cc: Sven Schnelle <svens@...ux.ibm.com>
> > Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > Cc: linux-kernel@...r.kernel.org
> > To: linux-s390@...r.kernel.org
> > ---
> >  arch/s390/kernel/irq.c | 11 +++++------
> >  1 file changed, 5 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/s390/kernel/irq.c b/arch/s390/kernel/irq.c
> > index 3033f616e256..6302dc7874cf 100644
> > --- a/arch/s390/kernel/irq.c
> > +++ b/arch/s390/kernel/irq.c
> > @@ -205,12 +205,13 @@ static void show_msi_interrupt(struct seq_file *p, int irq)
> >  	unsigned long flags;
> >  	int cpu;
> >  
> > -	irq_lock_sparse();
> > +	raw_spin_lock_irqsave(&desc->lock, flags);
> >  	desc = irq_to_desc(irq);
> 
> How is this supposed to work? desc get's initialized after its random
> stack value has been used as a pointer to lock something...

Oops. You are right. What about using rcu_read_lock() directly?


diff --git a/arch/s390/kernel/irq.c b/arch/s390/kernel/irq.c
index 3033f616e256..45393919fe61 100644
--- a/arch/s390/kernel/irq.c
+++ b/arch/s390/kernel/irq.c
@@ -205,7 +205,7 @@ static void show_msi_interrupt(struct seq_file *p, int irq)
 	unsigned long flags;
 	int cpu;
 
-	irq_lock_sparse();
+	rcu_read_lock();
 	desc = irq_to_desc(irq);
 	if (!desc)
 		goto out;
@@ -224,7 +224,7 @@ static void show_msi_interrupt(struct seq_file *p, int irq)
 	seq_putc(p, '\n');
 	raw_spin_unlock_irqrestore(&desc->lock, flags);
 out:
-	irq_unlock_sparse();
+	rcu_read_unlock();
 }
 
 /*


Thanks,

	Pingfan