linux-kernel - Re: [PATCH v2] coresight: Fix possible deadlock in coresight_panic

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Message-ID: <6d833ea6-328f-4743-abfa-fb09b168849e@linux.dev>
Date: Fri, 12 Sep 2025 10:38:44 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Yeoreum Yun <yeoreum.yun@....com>, Leo Yan <leo.yan@....com>
Cc: Suzuki K Poulose <suzuki.poulose@....com>, coresight@...ts.linaro.org,
 linux-arm-kernel@...ts.infradead.org,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Mike Leach <mike.leach@...aro.org>, Linu Cherian <lcherian@...vell.com>,
 James Clark <james.clark@...aro.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] coresight: Fix possible deadlock in coresight_panic_cb

On 9/12/25 07:03, Yeoreum Yun wrote:
> Hi,
> 
>> Hi Sean,
>>
>> On Thu, Sep 11, 2025 at 11:33:15AM -0400, Sean Anderson wrote:
>> > coresight_panic_cb is called with interrupts disabled during panics.
>> > However, bus_for_each_dev calls bus_to_subsys which takes
>> > bus_kset->list_lock without disabling IRQs. This will cause a deadlock
>> > if a panic occurs while one of the other coresight functions that uses
>> > bus_for_each_dev is running.
>>
>> The decription is a bit misleading. Even when IRQ is disabled, if an
>> exception happens, a CPU still can be trapped for handling kernel panic.
>>
>> > Maintain a separate list of coresight devices to access during a panic.
>>
>> Rather than maintaining a separate list and introducing a new spinlock,
>> I would argue if we can simply register panic notifier in TMC ETR and
>> ETF drviers (see tmc_panic_sync_etr() and tmc_panic_sync_etf()).
>>
>> If there is no dependency between CoreSight modules in panic sync flow,
>> it is not necessary to maintain list (and lock) for these modules.

Yeah, I was thinking about this as I was preparing v2 of this patch.

> +1 for this.
> and using the spinlock in the panic_cb doesn't work on PREEMPT_RT side.

What do you mean by this? I am using lockdep and it did not warn about this,
so I assume that on PREEMPT_RT IRQs remain enabled in this path.

--Sean