1 files changed, 38 insertions, 6 deletions

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 32313c084442..0f0d4704ddd8 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -985,6 +985,11 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 
                 /* 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_desc *desc, struct irqaction *new)
         }
 
         /*
-         * 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);