[PATCH v2] refscale: Fix use of uninitalized wait_queue_head_t

[Waiman Long <longman@redhat.com>]

It was found that running the refscale test might crash the kernel once
in a while with the following error:

[ 8569.952896] BUG: unable to handle page fault for address: ffffffffffffffe8
[ 8569.952900] #PF: supervisor read access in kernel mode
[ 8569.952902] #PF: error_code(0x0000) - not-present page
[ 8569.952904] PGD c4b048067 P4D c4b049067 PUD c4b04b067 PMD 0
[ 8569.952910] Oops: 0000 [#1] PREEMPT_RT SMP NOPTI
[ 8569.952916] Hardware name: Dell Inc. PowerEdge R750/0WMWCR, BIOS 1.2.4 05/28/2021
[ 8569.952917] RIP: 0010:prepare_to_wait_event+0x101/0x190
  :
[ 8569.952940] Call Trace:
[ 8569.952941]  <TASK>
[ 8569.952944]  ref_scale_reader+0x380/0x4a0 [refscale]
[ 8569.952959]  kthread+0x10e/0x130
[ 8569.952966]  ret_from_fork+0x1f/0x30
[ 8569.952973]  </TASK>

This is likely caused by the fact that init_waitqueue_head() is
called after the ref_scale_reader kthread is created. The kthread
can potentially try to use the waitqueue head before it is properly
initialized. The crash happened at

	static inline void __add_wait_queue(...)
	{
		:
		if (!(wq->flags & WQ_FLAG_PRIORITY)) <=== Crash here

The offset of flags from list_head entry in wait_queue_entry is -0x18. If
reader_tasks[i].wq.head.next is NULL as allocated reader_task structure
is zero initialized, the instruction will try to access address
0xffffffffffffffe8 which is the fault address listed above.

Fix this by initializing the waitqueue head first before kthread
creation.

Fixes: 653ed64b01dc ("refperf: Add a test to measure performance of read-side synchronization")
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
---
 kernel/rcu/refscale.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
index 1970ce5f22d4..71d138573856 100644
--- a/kernel/rcu/refscale.c
+++ b/kernel/rcu/refscale.c
@@ -1107,12 +1107,11 @@ ref_scale_init(void)
 	VERBOSE_SCALEOUT("Starting %d reader threads", nreaders);
 
 	for (i = 0; i < nreaders; i++) {
+		init_waitqueue_head(&reader_tasks[i].wq);
 		firsterr = torture_create_kthread(ref_scale_reader, (void *)i,
 						  reader_tasks[i].task);
 		if (torture_init_error(firsterr))
 			goto unwind;
-
-		init_waitqueue_head(&(reader_tasks[i].wq));
 	}
 
 	// Main Task
-- 
2.31.1