2020-04-22 04:13:29

by Davidlohr Bueso

Subject: [PATCH 3/5] rcuwait: Introduce prepare_to and finish_rcuwait

This allows further flexibility for some callers to implement
ad-hoc versions of the generic rcuwait_wait_event(). For example,
kvm will need this to maintain tracing semantics. The naming
is of course similar to what waitqueue apis offer.

Signed-off-by: Davidlohr Bueso <[email protected]>
---
include/linux/rcuwait.h | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/include/linux/rcuwait.h b/include/linux/rcuwait.h
index 6ebb23258a27..0c6a3d0d25ab 100644
--- a/include/linux/rcuwait.h
+++ b/include/linux/rcuwait.h
@@ -29,12 +29,25 @@ extern int rcuwait_wake_up(struct rcuwait *w);

/*
* The caller is responsible for locking around rcuwait_wait_event(),
- * such that writes to @task are properly serialized.
+ * and prepare_to_rcuwait() such that writes to @task are properly
+ * serialized.
*/
+
+static inline void prepare_to_rcuwait(struct rcuwait *w)
+{
+ rcu_assign_pointer(w->task, current);
+}
+
+static inline void finish_rcuwait(struct rcuwait *w)
+{
+ WRITE_ONCE(w->task, NULL);
+ __set_current_state(TASK_RUNNING);
+}
+
#define rcuwait_wait_event(w, condition, state) \
({ \
int __ret = 0; \
- rcu_assign_pointer((w)->task, current); \
+ prepare_to_rcuwait(w); \
for (;;) { \
/* \
* Implicit barrier (A) pairs with (B) in \
@@ -51,9 +64,7 @@ extern int rcuwait_wake_up(struct rcuwait *w);
\
schedule(); \
} \
- \
- WRITE_ONCE((w)->task, NULL); \
- __set_current_state(TASK_RUNNING); \
+ finish_rcuwait(w); \
__ret; \
})

--
2.16.4

2020-04-23 09:25:55

by Peter Zijlstra

Subject: Re: [PATCH 3/5] rcuwait: Introduce prepare_to and finish_rcuwait

On Tue, Apr 21, 2020 at 09:07:37PM -0700, Davidlohr Bueso wrote:

> +static inline void prepare_to_rcuwait(struct rcuwait *w)
> +{
> + rcu_assign_pointer(w->task, current);
> +}
> +
> +static inline void finish_rcuwait(struct rcuwait *w)
> +{
> + WRITE_ONCE(w->task, NULL);

I think that wants to be:

rcu_assign_pointer(w->task, NULL);

There is a special case in rcu_assign_pointer() that looses the barrier,
but it will keep the __rcu sparse people happy. That is w->task is
__rcu, and WRITE_ONCE ignores that etc.. blah.

The alternative is using RCU_INIT_POINTER() I suppose.

> + __set_current_state(TASK_RUNNING);
> +}