genirq: Prepare the handling of shared oneshot interrupts

For level type interrupts we need to track how many threads are on
flight to avoid useless interrupt storms when not all thread handlers
have finished yet. Keep track of the woken threads and only unmask
when there are no more threads in flight.

Yes, I'm lazy and using a bitfield. But not only because I'm lazy, the
main reason is that it's way simpler than using a refcount. A refcount
based solution would need to keep track of various things like
crashing the irq thread, spurious interrupts coming in,
disables/enables, free_irq() and some more. The bitfield keeps the
tracking simple and makes things just work. It's also nicely confined
to the thread code pathes and does not require additional checks all
over the place.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20110223234956.388095876@linutronix.de>

1 files changed, 48 insertions, 6 deletions

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 01f8a9519e63..2301de19ac7d 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -617,8 +617,11 @@ static int irq_wait_for_interrupt(struct irqaction *action)
  * handler finished. unmask if the interrupt has not been disabled and
  * is marked MASKED.
  */
-static void irq_finalize_oneshot(unsigned int irq, struct irq_desc *desc)
+static void irq_finalize_oneshot(struct irq_desc *desc,
+                                 struct irqaction *action, bool force)
 {
+        if (!(desc->istate & IRQS_ONESHOT))
+                return;
 again:
         chip_bus_lock(desc);
         raw_spin_lock_irq(&desc->lock);
@@ -631,6 +634,11 @@ again:
          * on the other CPU. If we unmask the irq line then the
          * interrupt can come in again and masks the line, leaves due
          * to IRQS_INPROGRESS and the irq line is masked forever.
+         *
+         * This also serializes the state of shared oneshot handlers
+         * versus "desc->threads_onehsot |= action->thread_mask;" in
+         * irq_wake_thread(). See the comment there which explains the
+         * serialization.
          */
         if (unlikely(desc->istate & IRQS_INPROGRESS)) {
                 raw_spin_unlock_irq(&desc->lock);
@@ -639,11 +647,23 @@ again:
                 goto again;
         }
 
-        if (!(desc->istate & IRQS_DISABLED) && (desc->istate & IRQS_MASKED)) {
+        /*
+         * Now check again, whether the thread should run. Otherwise
+         * we would clear the threads_oneshot bit of this thread which
+         * was just set.
+         */
+        if (!force && test_bit(IRQTF_RUNTHREAD, &action->thread_flags))
+                goto out_unlock;
+
+        desc->threads_oneshot &= ~action->thread_mask;
+
+        if (!desc->threads_oneshot && !(desc->istate & IRQS_DISABLED) &&
+            (desc->istate & IRQS_MASKED)) {
                 irq_compat_clr_masked(desc);
                 desc->istate &= ~IRQS_MASKED;
                 desc->irq_data.chip->irq_unmask(&desc->irq_data);
         }
+out_unlock:
         raw_spin_unlock_irq(&desc->lock);
         chip_bus_sync_unlock(desc);
 }
@@ -691,7 +711,7 @@ static int irq_thread(void *data)
         };
         struct irqaction *action = data;
         struct irq_desc *desc = irq_to_desc(action->irq);
-        int wake, oneshot = desc->istate & IRQS_ONESHOT;
+        int wake;
 
         sched_setscheduler(current, SCHED_FIFO, &param);
         current->irqaction = action;
@@ -719,8 +739,7 @@ static int irq_thread(void *data)
 
                         action->thread_fn(action->irq, action->dev_id);
 
-                        if (oneshot)
-                                irq_finalize_oneshot(action->irq, desc);
+                        irq_finalize_oneshot(desc, action, false);
                 }
 
                 wake = atomic_dec_and_test(&desc->threads_active);
@@ -729,6 +748,9 @@ static int irq_thread(void *data)
                         wake_up(&desc->wait_for_threads);
         }
 
+        /* Prevent a stale desc->threads_oneshot */
+        irq_finalize_oneshot(desc, action, true);
+
         /*
          * Clear irqaction. Otherwise exit_irq_thread() would make
          * fuzz about an active irq thread going into nirvana.
@@ -743,6 +765,7 @@ static int irq_thread(void *data)
 void exit_irq_thread(void)
 {
         struct task_struct *tsk = current;
+        struct irq_desc *desc;
 
         if (!tsk->irqaction)
                 return;
@@ -751,6 +774,14 @@ void exit_irq_thread(void)
                "exiting task \"%s\" (%d) is an active IRQ thread (irq %d)\n",
                tsk->comm ? tsk->comm : "", tsk->pid, tsk->irqaction->irq);
 
+        desc = irq_to_desc(tsk->irqaction->irq);
+
+        /*
+         * Prevent a stale desc->threads_oneshot. Must be called
+         * before setting the IRQTF_DIED flag.
+         */
+        irq_finalize_oneshot(desc, tsk->irqaction, true);
+
         /*
          * Set the THREAD DIED flag to prevent further wakeups of the
          * soon to be gone threaded handler.
@@ -767,7 +798,7 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 {
         struct irqaction *old, **old_ptr;
         const char *old_name = NULL;
-        unsigned long flags;
+        unsigned long flags, thread_mask = 0;
         int ret, nested, shared = 0;
         cpumask_var_t mask;
 
@@ -865,12 +896,23 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 
                 /* add new interrupt at end of irq queue */
                 do {
+                        thread_mask |= old->thread_mask;
                         old_ptr = &old->next;
                         old = *old_ptr;
                 } while (old);
                 shared = 1;
         }
 
+        /*
+         * Setup the thread mask for this irqaction. Unlikely to have
+         * 32 resp 64 irqs sharing one line, but who knows.
+         */
+        if (new->flags & IRQF_ONESHOT && thread_mask == ~0UL) {
+                ret = -EBUSY;
+                goto out_mask;
+        }
+        new->thread_mask = 1 << ffz(thread_mask);
+
         if (!shared) {
                 irq_chip_set_defaults(desc->irq_data.chip);