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

[Waiman Long <longman@redhat.com>]

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@redhat.com>
>
> 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.

Cheers,
Longman