linux-kernel - Re: [PATCH v3 2/3] softirq: Provide a handshake for canceling tasklets via polling

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Message-ID: <20250920012910.7265-1-hdanton@sina.com>
Date: Sat, 20 Sep 2025 09:29:07 +0800
From: Hillf Danton <hdanton@...a.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-rt-devel@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	Takashi Iwai <tiwai@...e.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v3 2/3] softirq: Provide a handshake for canceling tasklets via polling

On Thu, 18 Sep 2025 17:49:37 +0200 Sebastian Andrzej Siewior wrote:
>On 2025-09-18 21:47:52 [+0800], Hillf Danton wrote:
>> On Mon, 15 Sep 2025 09:39:33 +0200 Sebastian Andrzej Siewior wrote:
>> >On 2025-09-05 18:15:01 [+0800], Hillf Danton wrote:
>> >> 	CPU0			CPU1
>> >> 	----			----
>> >> 	lock A
>> >> 				tasklet C callback
>> >> 				lock A
>> >> 	cancel tasklet B
>> >> 	DEADLOCK-01
>> >> 
>> >> After this work could DEADLOCK-01 be triggered, given no chance for DEADLOCK-02 ?
>> >> 
>> >> 	CPU2			CPU3
>> >> 	----			----
>> >> 	lock A
>> >> 				timer C callback
>> >> 				lock A
>> >> 	timer_delete_sync(timer B)
>> >> 	DEADLOCK-02
>> >
>> > You are not supposed to acquire the lock, that is also acquired in the
>> > callback, while canceling the timer/ tasklet.
>> > Tell me please, how is this relevant?
>> > 
>> > If lock A is acquired on CPU0/ 2 then tasklet/ timer on CPU1/ 3 can't
>> > make progress. Now CPU0/ 2 waits for the callback to complete. This
>> > deadlocks as of today regardless of PREEMPT_RT and this change.
>> > 
>> In case of !RT, the chance for DEADLOCK-02 is zero because deadlock is
>> detected based on per-timer instead of per-cpu.
>
> But your "lock A" is global, isn't it?
>
IIUC whether lockA is global can be safely ignored here. DEADLOCK-02 can not
be detected with !RT because by define the callback of timerB has nothing to
do with timerC in addition to the current per-timer detecting mechanism.

This work however adds per-cpu detecting mechanism that fails to tell the
difference between timerB and timerC, thus false positive result comes. For
example the callback of timerB does not acquire lockA.