[PATCH 02/11] wait: Provide infrastructure to deal with nested blocking

Peter Zijlstra
2014-09-24T08:33:51Z

[-- Attachment #1: peterz-wait-nested.patch --]
[-- Type: text/plain, Size: 3854 bytes --]

There are a few places that call blocking primitives from wait loops,
provide infrastructure to support this without the typical
task_struct::state collision.

We record the wakeup in wait_queue_t::flags which leaves
task_struct::state free to be used by others.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/wait.h | 7 +++++
kernel/sched/wait.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 67 insertions(+), 1 deletion(-)

--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -13,9 +13,12 @@ typedef struct __wait_queue wait_queue_t
typedef int (*wait_queue_func_t)(wait_queue_t *wait, unsigned mode, int flags, void *key);
int default_wake_function(wait_queue_t *wait, unsigned mode, int flags, void *key);

+/* __wait_queue::flags */
+#define WQ_FLAG_EXCLUSIVE 0x01
+#define WQ_FLAG_WOKEN 0x02
+
struct __wait_queue {
unsigned int flags;
-#define WQ_FLAG_EXCLUSIVE 0x01
void *private;
wait_queue_func_t func;
struct list_head task_list;
@@ -829,6 +832,8 @@ void prepare_to_wait_exclusive(wait_queu
long prepare_to_wait_event(wait_queue_head_t *q, wait_queue_t *wait, int state);
void finish_wait(wait_queue_head_t *q, wait_queue_t *wait);
void abort_exclusive_wait(wait_queue_head_t *q, wait_queue_t *wait, unsigned int mode, void *key);
+long wait_woken(wait_queue_t *wait, unsigned mode, long timeout);
+int woken_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key);
int autoremove_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key);
int wake_bit_function(wait_queue_t *wait, unsigned mode, int sync, void *key);

--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -297,6 +297,67 @@ int autoremove_wake_function(wait_queue_
}
EXPORT_SYMBOL(autoremove_wake_function);

+
+/*
+ * DEFINE_WAIT_FUNC(wait, woken_wait_func);
+ *
+ * add_wait_queue(&wq, &wait);
+ * for (;;) {
+ * if (condition)
+ * break;
+ *
+ * p->state = mode; condition = true;
+ * smp_mb(); // A smp_wmb(); // C
+ * if (!wait->flags & WQ_FLAG_WOKEN) wait->flags |= WQ_FLAG_WOKEN;
+ * schedule() try_to_wake_up();
+ * p->state = TASK_RUNNING; ~~~~~~~~~~~~~~~~~~
+ * wait->flags &= ~WQ_FLAG_WOKEN; condition = true;
+ * smp_mb() // B smp_wmb(); // C
+ * wait->flags |= WQ_FLAG_WOKEN;
+ * }
+ * remove_wait_queue(&wq, &wait);
+ *
+ */
+long wait_woken(wait_queue_t *wait, unsigned mode, long timeout)
+{
+ set_current_state(mode); /* A */
+ /*
+ * The above implies an smp_mb(), which matches with the smp_wmb() from
+ * woken_wake_function() such that if we observe WQ_FLAG_WOKEN we must
+ * also observe all state before the wakeup.
+ */
+ if (!(wait->flags & WQ_FLAG_WOKEN))
+ timeout = schedule_timeout(timeout);
+ __set_current_state(TASK_RUNNING);
+
+ /*
+ * The below implies an smp_mb(), it too pairs with the smp_wmb() from
+ * woken_wake_function() such that we must either observe the wait
+ * condition being true _OR_ WQ_FLAG_WOKEN such that we will not miss
+ * an event.
+ */
+ set_mb(wait->flags, wait->flags & ~WQ_FLAG_WOKEN); /* B */
+
+ return timeout;
+}
+EXPORT_SYMBOL(wait_woken);
+
+int woken_wake_function(wait_queue_t *wait, unsigned mode, int sync, void *key)
+{
+ /*
+ * Although this function is called under waitqueue lock, LOCK
+ * doesn't imply write barrier and the users expects write
+ * barrier semantics on wakeup functions. The following
+ * smp_wmb() is equivalent to smp_wmb() in try_to_wake_up()
+ * and is paired with set_mb() in wait_woken().
+ */
+ smp_wmb(); /* C */
+ wait->flags |= WQ_FLAG_WOKEN;
+
+ return default_wake_function(wait, mode, sync, key);
+}
+EXPORT_SYMBOL(woken_wake_function);
+
int wake_bit_function(wait_queue_t *wait, unsigned mode, int sync, void *arg)
{
struct wait_bit_key *key = arg;