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Commit 4a5f4d2f authored by Eric Dumazet's avatar Eric Dumazet Committed by Thomas Gleixner
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genirq: Use rcu in kstat_irqs_usr() 



Jeremy Dorfman identified mutex contention when multiple threads
parse /proc/stat concurrently.

Since commit 425a5072 ("genirq: Free irq_desc with rcu"),
kstat_irqs_usr() can be switched to rcu locking, which removes this mutex
contention.

show_interrupts() case will be handled in a separate patch.

Reported-by: default avatarJeremy Dorfman <jdorfman@google.com>
Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Willem de Bruijn <willemb@google.com>
Link: https://lkml.kernel.org/r/20180618125612.155057-1-edumazet@google.com
parent 9ffc59d5

kernel/irq/irqdesc.c

+7 −6
Original line number Diff line number Diff line
@@ -443,6 +443,7 @@ static void free_desc(unsigned int irq) 
	 * We free the descriptor, masks and stat fields via RCU. That

	 * allows demultiplex interrupts to do rcu based management of

	 * the child interrupts.

	 * This also allows us to use rcu in kstat_irqs_usr().

	 */

	call_rcu(&desc->rcu, delayed_free_desc);

}
@@ -928,17 +929,17 @@ unsigned int kstat_irqs(unsigned int irq) 
 * kstat_irqs_usr - Get the statistics for an interrupt

 * @irq:	The interrupt number

 *

 * Returns the sum of interrupt counts on all cpus since boot for

 * @irq. Contrary to kstat_irqs() this can be called from any

 * preemptible context. It's protected against concurrent removal of

 * an interrupt descriptor when sparse irqs are enabled.

 * Returns the sum of interrupt counts on all cpus since boot for @irq.

 * Contrary to kstat_irqs() this can be called from any context.

 * It uses rcu since a concurrent removal of an interrupt descriptor is

 * observing an rcu grace period before delayed_free_desc()/irq_kobj_release().

 */

unsigned int kstat_irqs_usr(unsigned int irq)

{

	unsigned int sum;



	irq_lock_sparse();

	rcu_read_lock();

	sum = kstat_irqs(irq);

	irq_unlock_sparse();

	rcu_read_unlock();

	return sum;

}