padata: Add some code comments
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
diff --git a/kernel/padata.c b/kernel/padata.c
index ec6b8b7..ca89dfb 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -88,7 +88,7 @@
local_bh_enable();
}
-/*
+/**
* padata_do_parallel - padata parallelization function
*
* @pinst: padata instance
@@ -152,6 +152,23 @@
}
EXPORT_SYMBOL(padata_do_parallel);
+/*
+ * padata_get_next - Get the next object that needs serialization.
+ *
+ * Return values are:
+ *
+ * A pointer to the control struct of the next object that needs
+ * serialization, if present in one of the percpu reorder queues.
+ *
+ * NULL, if all percpu reorder queues are empty.
+ *
+ * -EINPROGRESS, if the next object that needs serialization will
+ * be parallel processed by another cpu and is not yet present in
+ * the cpu's reorder queue.
+ *
+ * -ENODATA, if this cpu has to do the parallel processing for
+ * the next object.
+ */
static struct padata_priv *padata_get_next(struct parallel_data *pd)
{
int cpu, num_cpus, empty, calc_seq_nr;
@@ -173,7 +190,7 @@
/*
* Calculate the seq_nr of the object that should be
- * next in this queue.
+ * next in this reorder queue.
*/
overrun = 0;
calc_seq_nr = (atomic_read(&queue->num_obj) * num_cpus)
@@ -248,15 +265,36 @@
struct padata_queue *queue;
struct padata_instance *pinst = pd->pinst;
+ /*
+ * We need to ensure that only one cpu can work on dequeueing of
+ * the reorder queue the time. Calculating in which percpu reorder
+ * queue the next object will arrive takes some time. A spinlock
+ * would be highly contended. Also it is not clear in which order
+ * the objects arrive to the reorder queues. So a cpu could wait to
+ * get the lock just to notice that there is nothing to do at the
+ * moment. Therefore we use a trylock and let the holder of the lock
+ * care for all the objects enqueued during the holdtime of the lock.
+ */
if (!spin_trylock_bh(&pd->lock))
return;
while (1) {
padata = padata_get_next(pd);
+ /*
+ * All reorder queues are empty, or the next object that needs
+ * serialization is parallel processed by another cpu and is
+ * still on it's way to the cpu's reorder queue, nothing to
+ * do for now.
+ */
if (!padata || PTR_ERR(padata) == -EINPROGRESS)
break;
+ /*
+ * This cpu has to do the parallel processing of the next
+ * object. It's waiting in the cpu's parallelization queue,
+ * so exit imediately.
+ */
if (PTR_ERR(padata) == -ENODATA) {
del_timer(&pd->timer);
spin_unlock_bh(&pd->lock);
@@ -274,6 +312,11 @@
spin_unlock_bh(&pd->lock);
+ /*
+ * The next object that needs serialization might have arrived to
+ * the reorder queues in the meantime, we will be called again
+ * from the timer function if noone else cares for it.
+ */
if (atomic_read(&pd->reorder_objects)
&& !(pinst->flags & PADATA_RESET))
mod_timer(&pd->timer, jiffies + HZ);
@@ -318,7 +361,7 @@
local_bh_enable();
}
-/*
+/**
* padata_do_serial - padata serialization function
*
* @padata: object to be serialized.
@@ -348,6 +391,7 @@
}
EXPORT_SYMBOL(padata_do_serial);
+/* Allocate and initialize the internal cpumask dependend resources. */
static struct parallel_data *padata_alloc_pd(struct padata_instance *pinst,
const struct cpumask *cpumask)
{
@@ -417,6 +461,7 @@
kfree(pd);
}
+/* Flush all objects out of the padata queues. */
static void padata_flush_queues(struct parallel_data *pd)
{
int cpu;
@@ -440,6 +485,7 @@
BUG_ON(atomic_read(&pd->refcnt) != 0);
}
+/* Replace the internal control stucture with a new one. */
static void padata_replace(struct padata_instance *pinst,
struct parallel_data *pd_new)
{
@@ -457,7 +503,7 @@
pinst->flags &= ~PADATA_RESET;
}
-/*
+/**
* padata_set_cpumask - set the cpumask that padata should use
*
* @pinst: padata instance
@@ -507,7 +553,7 @@
return 0;
}
-/*
+/**
* padata_add_cpu - add a cpu to the padata cpumask
*
* @pinst: padata instance
@@ -545,7 +591,7 @@
return 0;
}
-/*
+/**
* padata_remove_cpu - remove a cpu from the padata cpumask
*
* @pinst: padata instance
@@ -568,7 +614,7 @@
}
EXPORT_SYMBOL(padata_remove_cpu);
-/*
+/**
* padata_start - start the parallel processing
*
* @pinst: padata instance to start
@@ -581,7 +627,7 @@
}
EXPORT_SYMBOL(padata_start);
-/*
+/**
* padata_stop - stop the parallel processing
*
* @pinst: padata instance to stop
@@ -648,7 +694,7 @@
}
#endif
-/*
+/**
* padata_alloc - allocate and initialize a padata instance
*
* @cpumask: cpumask that padata uses for parallelization
@@ -703,10 +749,10 @@
}
EXPORT_SYMBOL(padata_alloc);
-/*
+/**
* padata_free - free a padata instance
*
- * @ padata_inst: padata instance to free
+ * @padata_inst: padata instance to free
*/
void padata_free(struct padata_instance *pinst)
{