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Date:   Fri, 7 Jul 2023 12:52:52 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Waiman Long <longman@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Josh Triplett <josh@joshtriplett.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Zqiang <qiang.zhang1211@gmail.com>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] refscale: Fix use of uninitalized wait_queue_head_t
Message-ID: <1629fe9b-aecc-466e-9d3c-179f90d41602@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <b3a22928-6537-ebca-d9c2-82d2f940208f@redhat.com>
 <140FCD95-052F-4DA9-AA5A-374D417490A9@joelfernandes.org>
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On Fri, Jul 07, 2023 at 03:24:58PM -0400, Joel Fernandes wrote:
> 
> 
> > On Jul 7, 2023, at 10:56 AM, Waiman Long <longman@redhat.com> 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@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.
> 
> 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@joelfernandes.org>

Applied, thank you!

							Thanx, Paul