genirq: Update code comments wrt recycled thread_mask

Previously a race existed between __free_irq() and __setup_irq() wherein
the thread_mask of a just removed action could be handed out to a newly
added action and the freed irq thread would then tread on the oneshot
mask bit of the newly added irq thread in irq_finalize_oneshot():

time
 |  __free_irq()
 |    raw_spin_lock_irqsave(&desc->lock, flags);
 |    <remove action from linked list>
 |    raw_spin_unlock_irqrestore(&desc->lock, flags);
 |
 |  __setup_irq()
 |    raw_spin_lock_irqsave(&desc->lock, flags);
 |    <traverse linked list to determine oneshot mask bit>
 |    raw_spin_unlock_irqrestore(&desc->lock, flags);
 |
 |  irq_thread() of freed irq (__free_irq() waits in synchronize_irq())
 |    irq_thread_fn()
 |      irq_finalize_oneshot()
 |        raw_spin_lock_irq(&desc->lock);
 |        desc->threads_oneshot &= ~action->thread_mask;
 |        raw_spin_unlock_irq(&desc->lock);
 v

The race was known at least since 2012 when it was documented in a code
comment by commit e04268b0effc ("genirq: Remove paranoid warnons and bogus
fixups"). The race itself is harmless as nothing touches any of the
potentially freed data after synchronize_irq().

In 2017 the race was close by commit 9114014cf4e6 ("genirq: Add mutex to
irq desc to serialize request/free_irq()"), apparently inadvertantly so
because the race is neither mentioned in the commit message nor was the
code comment updated.  Make up for that.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>
Cc: linux-pci@vger.kernel.org
Link: https://lkml.kernel.org/r/32fc25aa35ecef4b2692f57687bb7fc2a57230e2.1529828292.git.lukas@wunner.de

kernel/irq/manage.c

@@ -1025,10 +1025,7 @@ static int irq_thread(void *data)
 	 * This is the regular exit path. __free_irq() is stopping the
 	 * thread via kthread_stop() after calling
 	 * synchronize_irq(). So neither IRQTF_RUNTHREAD nor the
-	 * oneshot mask bit can be set. We cannot verify that as we
-	 * cannot touch the oneshot mask at this point anymore as
-	 * __setup_irq() might have given out currents thread_mask
-	 * again.
+	 * oneshot mask bit can be set.
 	 */
 	task_work_cancel(current, irq_thread_dtor);
 	return 0;
@@ -1245,7 +1242,9 @@ __setup_irq(unsigned int irq, struct irq_desc *desc, struct irqaction *new)
 	/*
 	 * Protects against a concurrent __free_irq() call which might wait
 	 * for synchronize_irq() to complete without holding the optional
-	 * chip bus lock and desc->lock.
+	 * chip bus lock and desc->lock. Also protects against handing out
+	 * a recycled oneshot thread_mask bit while it's still in use by
+	 * its previous owner.
 	 */
 	mutex_lock(&desc->request_mutex);