[dm-devel] [PATCH v2] swait: export symbols __prepare_to_swait and __finish_swait

[Mike Snitzer]

[Peter, in this v2 I switched to using _GPL for the exports and updated
the patch header.  As covered in previous mail, please let me know if
you're OK with me staging this change for 4.18 via linux-dm.git with
your Ack, thanks]

From: Mikulas Patocka <mpatocka at redhat.com>
Subject: [PATCH] swait: export symbols __prepare_to_swait and __finish_swait

__prepare_to_swait and __finish_swait are declared in
include/linux/swait.h but they are not exported; so they are not useable
from kernel modules.

A new consumer of swait (in dm-writecache) reduces its locking overhead
by using the spinlock in swait_queue_head to protect not only the wait
queue, but also the list of events.  Consequently, this swait consuming
kernel module needs to use these unlocked functions.

Peter Zijlstra explained:
  "The reason swait exists is to be deterministic (for RT) -- something
  that regular wait code cannot be.
  And by (ab)using / exporting the wait internal lock you risk losing
  that. So I don't think the proposed [dm-writecache] usage is bad, it
  is possible to create badness.
  So if we're going to export them; someone needs to keep an eye on things
  and ensure the lock isn't abused."

So while this new use of the wait internal lock doesn't jeopardize the
realtime requirements of swait, these exports do open swait's internal
locking up to being abused.  As such, EXPORT_SYMBOL_GPL is used because
any future consumers of __prepare_to_swait and __finish_swait must
always be thoroughly scrutinized.

Cc: Peter Zijlstra <peterz at infradead.org>
Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
Signed-off-by: Mike Snitzer <snitzer at redhat.com>
---
 kernel/sched/swait.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c
index b6fb2c3b3ff7..5d891b65ada5 100644
--- a/kernel/sched/swait.c
+++ b/kernel/sched/swait.c
@@ -75,6 +75,7 @@ void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait)
 	if (list_empty(&wait->task_list))
 		list_add(&wait->task_list, &q->task_list);
 }
+EXPORT_SYMBOL_GPL(__prepare_to_swait);
 
 void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state)
 {
@@ -104,6 +105,7 @@ void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
 	if (!list_empty(&wait->task_list))
 		list_del_init(&wait->task_list);
 }
+EXPORT_SYMBOL_GPL(__finish_swait);
 
 void finish_swait(struct swait_queue_head *q, struct swait_queue *wait)
 {
-- 
2.15.0