Re: [PATCH] refscale: Fix use of uninitalized wait_queue_head_t

[Waiman Long <longman@redhat.com>]

On 7/7/23 03:27, Zhuo, Qiuxu wrote:
>> From: Waiman Long <longman@redhat.com>
>> ...
>> Subject: [PATCH] refscale: Fix use of uninitalized wait_queue_head_t
>>
>> It was found that running the refscale test might sometimes crash the kernel
>> 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. So the kthread may try to use the
>> waitqueue head before it is properly initialized. 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>
>> ---
>>   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..e365d6f8c139 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));
> Running checkpatch.pl tool with the " --strict" option, it complained that
> "CHECK: Unnecessary parentheses around reader_tasks[i].wq".
> I know that you just moved the code position. The tool should have
> complained the original code. 😊
>
> Other than that, this patch LGTM.
>
>      Reviewed-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
>
> Thanks!
> -Qiuxu

Thanks for the review. I will do --strict test next time.

Cheers,
Longman

>
>>   		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