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

[Paul E. McKenney]

On Fri, Jul 07, 2023 at 03:24:58PM -0400, Joel Fernandes wrote:
> 
> 
> > 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>

Applied, thank you!

							Thanx, Paul