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Date:   Thu, 12 Oct 2023 17:39:08 +0200
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Subject: Re: [PATCH] Subject: thermal: Fix potential race condition in
 suspend/resume
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To:     Bo Ye <bo.ye@mediatek.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Amit Kucheria <amitk@kernel.org>,
        Zhang Rui <rui.zhang@intel.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
Cc:     yugang.wang@mediatek.com, yongdong.zhang@mediatek.com,
        browse.zhang@mediatek.com, linux-pm@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org
References: <20230916113327.85693-1-bo.ye@mediatek.com>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
In-Reply-To: <20230916113327.85693-1-bo.ye@mediatek.com>
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Precedence: bulk

On 16/09/2023 13:33, Bo Ye wrote:
> From: "yugang.wang" <yugang.wang@mediatek.com>
> 
> Body:
> This patch fixes a race condition during system resume. It occurs if
> the system is exiting a suspend state and a user is trying to
> register/unregister a thermal zone concurrently. The root cause is
> that both actions access the `thermal_tz_list`.

I'm not sure the tasks are already thawed during POST_RESTORE, so no 
user can unload a driver and then reaching the race window.

Is that an observed issue?


> In detail:
> 
> 1. At PM_POST_SUSPEND during the resume, the system reads all thermal
>     zones in `thermal_tz_list`, then resets and updates their
>     temperatures.
> 2. When registering/unregistering a thermal zone, the
>     `thermal_tz_list` gets manipulated.
> 
> These two actions might occur concurrently, causing a race condition.
> To solve this issue, we introduce a mutex lock to protect
> `thermal_tz_list` from being modified while it's being read and
> updated during the resume from suspend.
> 
> Kernel oops excerpt related to this fix:
> 
> [ 5201.869845] [T316822] pc: [0xffffffeb7d4876f0] mutex_lock+0x34/0x170
> [ 5201.869856] [T316822] lr: [0xffffffeb7ca98a84] thermal_pm_notify+0xd4/0x26c
> [... cut for brevity ...]
> [ 5201.871061] [T316822]  suspend_prepare+0x150/0x470
> [ 5201.871067] [T316822]  enter_state+0x84/0x6f4
> [ 5201.871076] [T316822]  state_store+0x15c/0x1e8
> 
> Change-Id: Ifdbdecba17093f91eab7e36ce04b46d311ca6568
> Signed-off-by: yugang.wang <yugang.wang@mediatek.com>
> Signed-off-by: Bo Ye <bo.ye@mediatek.com>
> ---
>   drivers/thermal/thermal_core.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/thermal/thermal_core.c b/drivers/thermal/thermal_core.c
> index 8717a3343512..a7a18ed57b6d 100644
> --- a/drivers/thermal/thermal_core.c
> +++ b/drivers/thermal/thermal_core.c
> @@ -1529,12 +1529,14 @@ static int thermal_pm_notify(struct notifier_block *nb,
>   	case PM_POST_HIBERNATION:
>   	case PM_POST_RESTORE:
>   	case PM_POST_SUSPEND:
> +		mutex_lock(&thermal_list_lock);
>   		atomic_set(&in_suspend, 0);
>   		list_for_each_entry(tz, &thermal_tz_list, node) {
>   			thermal_zone_device_init(tz);
>   			thermal_zone_device_update(tz,
>   						   THERMAL_EVENT_UNSPECIFIED);
>   		}
> +		mutex_unlock(&thermal_list_lock);
>   		break;
>   	default:
>   		break;

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