Re: [PATCH 01/10] random: make 'add_interrupt_randomness()' do something sane

["Theodore Ts'o" <tytso@mit.edu>]

On Thu, Jul 05, 2012 at 02:47:52PM -0700, Linus Torvalds wrote:
> On Thu, Jul 5, 2012 at 2:39 PM, Matt Mackall <mpm@selenic.com> wrote:
> >
> > From my read, this code path gets called on timer interrupts too.
> 
> That's hopefully never true for any normal cases (timers are *very*
> special - they tend to go through their own architecture-specific
> stuff). On modern PC's, for example, the timers happen through the
> local apic timers directly.
> 
> In fact, with SMP, it's a really bad idea to use a normal irq for
> timer interrupts, since you really really want per-CPU-core timers.
> 
> But yes, we should probably make sure that *if* the architecture uses
> regular interrupts for timers we don't count them. The problematic
> embedded platforms are often pretty crap hardware: even when they are
> SMP, they might use an external timer irq (and then we broadcast the
> thing). I think we have the IRQF_TIMER flag for that, so we could add
> a check for that.

Like this?

						- Ted

diff --git a/kernel/irq/handle.c b/kernel/irq/handle.c
index 8270db0..0ce4863 100644
--- a/kernel/irq/handle.c
+++ b/kernel/irq/handle.c
@@ -133,7 +133,7 @@ irqreturn_t
 handle_irq_event_percpu(struct irq_desc *desc, struct irqaction *action)
 {
 	irqreturn_t retval = IRQ_NONE;
-	unsigned int random = 0, irq = desc->irq_data.irq;
+	unsigned int random = 1, irq = desc->irq_data.irq;
 
 	do {
 		irqreturn_t res;
@@ -161,7 +161,8 @@ handle_irq_event_percpu(struct irq_desc *desc, struct irqaction *action)
 
 			/* Fall through to add to randomness */
 		case IRQ_HANDLED:
-			random |= action->flags;
+			if (action->flags & __IRQF_TIMER)
+				random = 0;
 			break;
 
 		default:
@@ -172,7 +173,8 @@ handle_irq_event_percpu(struct irq_desc *desc, struct irqaction *action)
 		action = action->next;
 	} while (action);
 
-	add_interrupt_randomness(irq);
+	if (random)
+		add_interrupt_randomness(irq);
 
 	if (!noirqdebug)
 		note_interrupt(irq, desc, retval);