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Date:   Fri, 21 May 2021 10:18:53 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Huangzhaoyang <huangzhaoyang@gmail.com>
Cc:     Suren Baghdasaryan <surenb@google.com>,
        Zhaoyang Huang <zhaoyang.huang@unisoc.com>,
        Ziwei Dai <ziwei.dai@unisoc.com>, Ke Wang <ke.wang@unisoc.com>,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH v6] psi: fix race between psi_trigger_create/destroy
Message-ID: <YKfBTWQxPkI5jKUe@cmpxchg.org>
References: <1621562754-8158-1-git-send-email-huangzhaoyang@gmail.com>
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On Fri, May 21, 2021 at 10:05:54AM +0800, Huangzhaoyang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> 
> Race detected between psi_trigger_destroy/create as shown below, which
> cause panic by accessing invalid psi_system->poll_wait->wait_queue_entry
> and psi_system->poll_timer->entry->next. Under this modification, the
> race window is removed by initialising poll_wait and poll_timer in
> group_init which are executed only once at beginning.
> 
> psi_trigger_destroy                      psi_trigger_create
> mutex_lock(trigger_lock);
> rcu_assign_pointer(poll_task, NULL);
> mutex_unlock(trigger_lock);
> 					mutex_lock(trigger_lock);
> 					if (!rcu_access_pointer(group->poll_task)) {
> 
> 						timer_setup(poll_timer, poll_timer_fn, 0);
> 
> 						rcu_assign_pointer(poll_task, task);
> 					}
> 					mutex_unlock(trigger_lock);
> 
> synchronize_rcu();
> del_timer_sync(poll_timer); <-- poll_timer has been reinitialized by
> psi_trigger_create
> 
> So, trigger_lock/RCU correctly protects destruction of group->poll_task but
> misses this race affecting poll_timer and poll_wait.
> 
> Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger
> scheduling mechanism")
> 
> Signed-off-by: ziwei.dai <ziwei.dai@unisoc.com>
> Signed-off-by: ke.wang <ke.wang@unisoc.com>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>

Acked-by: Johannes Weiner <hannes@cmpxchg.org>