[PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi_interrupt()

[PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi_interrupt()

[Pingfan Liu]

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@gmail.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org
To: linux-s390@vger.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);
-	if (!desc)
-		goto out;
+	if (!desc) {
+		raw_spin_unlock_irqrestore(&desc->lock, flags);
+		return;
+	}
 
-	raw_spin_lock_irqsave(&desc->lock, flags);
 	seq_printf(p, "%3d: ", irq);
 	for_each_online_cpu(cpu)
 		seq_printf(p, "%10u ", irq_desc_kstat_cpu(desc, cpu));
@@ -223,8 +224,6 @@ 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();
 }
 
 /*
-- 
2.31.1

Re: [PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi_interrupt()

[Heiko Carstens]

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@gmail.com>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> Cc: Sven Schnelle <svens@linux.ibm.com>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: linux-kernel@vger.kernel.org
> To: linux-s390@vger.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...

Re: [PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi_interrupt()

[Pingfan Liu]

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@gmail.com>
> > Cc: Heiko Carstens <hca@linux.ibm.com>
> > Cc: Vasily Gorbik <gor@linux.ibm.com>
> > Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> > Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> > Cc: Sven Schnelle <svens@linux.ibm.com>
> > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > Cc: linux-kernel@vger.kernel.org
> > To: linux-s390@vger.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

Re: [PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi_interrupt()

[Heiko Carstens]

On Thu, Apr 21, 2022 at 11:36:17AM +0800, Pingfan Liu wrote:
> 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();

That looks like it should work. Please resend and also add a reference
to commit 74bdf7815dfb ("genirq: Speedup show_interrupts()") which
explains why this works.

Re: [PATCH 4/9] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi_interrupt()

[Pingfan Liu]

On Thu, Apr 21, 2022 at 01:42:59PM +0200, Heiko Carstens wrote:
> On Thu, Apr 21, 2022 at 11:36:17AM +0800, Pingfan Liu wrote:
> > 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();
> 
> That looks like it should work. Please resend and also add a reference
> to commit 74bdf7815dfb ("genirq: Speedup show_interrupts()") which
> explains why this works.

Thanks for your review. I will follow up with V2.

Regards,

	Pingfan

[PATCHv2] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi_interrupt()

[Pingfan Liu]

As demonstrated by commit 74bdf7815dfb ("genirq: Speedup show_interrupts()"),
irq_desc can be accessed safely in RCU read section.

Hence here resorting to rcu read lock to get rid of irq_lock_sparse().

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org
To: linux-s390@vger.kernel.org

---
 arch/s390/kernel/irq.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

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();
 }
 
 /*
-- 
2.31.1

Re: [PATCHv2] s390/irq: utilize RCU instead of irq_lock_sparse() in show_msi_interrupt()

[Heiko Carstens]

On Fri, Apr 22, 2022 at 06:02:12PM +0800, Pingfan Liu wrote:
> As demonstrated by commit 74bdf7815dfb ("genirq: Speedup show_interrupts()"),
> irq_desc can be accessed safely in RCU read section.
> 
> Hence here resorting to rcu read lock to get rid of irq_lock_sparse().
> 
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> Cc: Heiko Carstens <hca@linux.ibm.com>
> Cc: Vasily Gorbik <gor@linux.ibm.com>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
> Cc: Sven Schnelle <svens@linux.ibm.com>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: linux-kernel@vger.kernel.org
> To: linux-s390@vger.kernel.org
> 
> ---
>  arch/s390/kernel/irq.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Applied, thanks!