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Date:   Thu, 17 Aug 2023 10:59:26 +0100
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Subject: Re: [PATCH v2 1/2] coresight: trbe: Fix TRBE potential sleep in
 atomic context
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To:     Anshuman Khandual <anshuman.khandual@arm.com>, hejunhao3@huawei.com
Cc:     coresight@lists.linaro.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, jonathan.cameron@huawei.com,
        mike.leach@linaro.org, linuxarm@huawei.com, yangyicong@huawei.com,
        prime.zeng@hisilicon.com
References: <20230814093813.19152-1-hejunhao3@huawei.com>
 <20230816141008.535450-1-suzuki.poulose@arm.com>
 <80ef7a87-6adf-27bc-43ae-05ae5680e418@arm.com>
From:   Suzuki K Poulose <suzuki.poulose@arm.com>
In-Reply-To: <80ef7a87-6adf-27bc-43ae-05ae5680e418@arm.com>
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On 17/08/2023 08:13, Anshuman Khandual wrote:
> Hello Junhao,
> 
> On 8/16/23 19:40, Suzuki K Poulose wrote:
>> From: Junhao He <hejunhao3@huawei.com>
>>
>> smp_call_function_single() will allocate an IPI interrupt vector to
>> the target processor and send a function call request to the interrupt
>> vector. After the target processor receives the IPI interrupt, it will
>> execute arm_trbe_remove_coresight_cpu() call request in the interrupt
>> handler.
>>
>> According to the device_unregister() stack information, if other process
>> is useing the device, the down_write() may sleep, and trigger deadlocks
>> or unexpected errors.
>>
>>    arm_trbe_remove_coresight_cpu
>>      coresight_unregister
>>        device_unregister
>>          device_del
>>            kobject_del
>>              __kobject_del
>>                sysfs_remove_dir
>>                  kernfs_remove
>>                    down_write ---------> it may sleep
> 
> But how did you really detect this problem ? Does this show up as an warning when
> you enable lockdep debug ? OR it really happened during a real workload execution
> followed by TRBE module unload. Although the problem seems plausible (which needs
> fixing), just wondering how did we trigger this.

I was able to trigger this with just :

modprobe coresight-trbe; modprobe -r coresight-trbe;

With all the bells and whistles enabled in the kernel.

Suzuki