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

[Joel Fernandes]

> On Jul 7, 2023, at 10:56 AM, Waiman Long <longman@xxxxxxxxxx> wrote:
> 
> On 7/7/23 10:07, Davidlohr Bueso wrote:
>>> 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.
> 
> Red Hat does have a pretty large QE organization that run all sort of tests include this one pretty frequently. The race window is pretty small, but they did hit this once in a while.

Having worked on this test initially, I am happy to hear that Redhat runs this test!

Thanks for fixing this issue.
Acked-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

-Joel

> 
> Cheers,
> Longman
>