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

[Davidlohr Bueso]

On Thu, 06 Jul 2023, Waiman Long wrote:

> 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>

Strange this wasn't reported sooner.

Reviewed-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>