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Date:   Fri, 7 Jul 2023 07:07:33 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Waiman Long <longman@redhat.com>
Cc:     "Paul E. McKenney" <paulmck@linux.ibm.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Neeraj Upadhyay <quic_neeraju@quicinc.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        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: <y5g37s3yhsqtmdoqziwnsd4kxtudlpqwv2iunt372abpmdzrry@43e54c374j4s>
References: <20230707000117.2371697-1-longman@redhat.com>
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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.

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>