linux-kernel - Re: [ 57/72] genirq: Unmask oneshot irqs when thread was not woken

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Date:	Tue, 6 Mar 2012 23:18:54 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Sven Joachim <svenjoac@....de>,
	Stefan Lippers-Hollmann <s.L-H@....de>,
	Greg KH <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Jonathan Nieder <jrnieder@...il.com>,
	linux-wireless@...r.kernel.org,
	Stefano Brivio <stefano.brivio@...imi.it>
Subject: Re: [ 57/72] genirq: Unmask oneshot irqs when thread was not woken

On Tue, 6 Mar 2012, Linus Torvalds wrote:
> On Tue, Mar 6, 2012 at 12:26 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> without ever doing any insane bit scanning.

Duh, yes. That's really insane. I first did

- new->thread_mask = 1 << ffz(thread_mask);
+ new->thread_mask = (new->flags & IRQF_ONESHOT) ? 1 << ffz(thread_mask) : 0;

and then decided to move it into the already existing new->flags &
IRQF_ONESHOT check section and deleted the wrong part ....

/me looks for a huge brown paperbag.

Thanks for catching it !

       tglx

-------------->
Subject: genirq: Clear action->thread_mask if IRQ_ONESHOT is not set

commit ac5637611(genirq: Unmask oneshot irqs when thread was not
woken) fails to unmask when a !IRQ_ONESHOT threaded handler is handled
by handle_level_irq. This happens because thread_mask is or'ed
unconditionally in irq_wake_thread(), but for !IRQ_ONESHOT interrupts
never cleared. So the check for !desc->thread_active fails and keeps
the interrupt disabled.

Keep the thread_mask zero for !IRQ_ONESHOT interrupts.

Document the thread_mask magic while at it.

Reported-by: Sven Joachim <svenjoac@....de>
Cc: stable@...r.kernel.org
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---

Index: tip/kernel/irq/manage.c
===================================================================
--- tip.orig/kernel/irq/manage.c
+++ tip/kernel/irq/manage.c
@@ -985,6 +985,11 @@ __setup_irq(unsigned int irq, struct irq
 
 		/* add new interrupt at end of irq queue */
 		do {
+			/*
+			 * Or all existing action->thread_mask bits,
+			 * so we can find the next zero bit for this
+			 * new action.
+			 */
 			thread_mask |= old->thread_mask;
 			old_ptr = &old->next;
 			old = *old_ptr;
@@ -993,14 +998,41 @@ __setup_irq(unsigned int irq, struct irq
 	}
 
 	/*
-	 * Setup the thread mask for this irqaction. Unlikely to have
-	 * 32 resp 64 irqs sharing one line, but who knows.
+	 * Setup the thread mask for this irqaction for ONESHOT. For
+	 * !ONESHOT irqs the thread mask is 0 so we can avoid a
+	 * conditional in irq_wake_thread().
 	 */
-	if (new->flags & IRQF_ONESHOT && thread_mask == ~0UL) {
-		ret = -EBUSY;
-		goto out_mask;
+	if (new->flags & IRQF_ONESHOT) {
+		/*
+		 * Unlikely to have 32 resp 64 irqs sharing one line,
+		 * but who knows.
+		 */
+		if (thread_mask == ~0UL) {
+			ret = -EBUSY;
+			goto out_mask;
+		}
+		/*
+		 * The thread_mask for the action is or'ed to
+		 * desc->thread_active to indicate that the
+		 * IRQF_ONESHOT thread handler has been woken, but not
+		 * yet finished. The bit is cleared when a thread
+		 * completes. When all threads of a shared interrupt
+		 * line have completed desc->threads_active becomes
+		 * zero and the interrupt line is unmasked. See
+		 * handle.c:irq_wake_thread() for further information.
+		 *
+		 * If no thread is woken by primary (hard irq context)
+		 * interrupt handlers, then desc->threads_active is
+		 * also checked for zero to unmask the irq line in the
+		 * affected hard irq flow handlers
+		 * (handle_[fasteoi|level]_irq).
+		 *
+		 * The new action gets the first zero bit of
+		 * thread_mask assigned. See the loop above which or's
+		 * all existing action->thread_mask bits.
+		 */
+		new->thread_mask = 1 << ffz(thread_mask);
 	}
-	new->thread_mask = 1 << ffz(thread_mask);
 
 	if (!shared) {
 		init_waitqueue_head(&desc->wait_for_threads);
--
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