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

[Zhuo, Qiuxu]

> From: Waiman Long <longman@xxxxxxxxxx>
> ...
> 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@xxxxxxxxxx>
> ---
>  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@xxxxxxxxx>

Thanks!
-Qiuxu

>  		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