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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Hillf Danton <hdanton@sina.com>
Cc: linux-rt-devel@lists.linux.dev, linux-kernel@vger.kernel.org,
	Takashi Iwai <tiwai@suse.de>, Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v3 2/3] softirq: Provide a handshake for canceling
 tasklets via polling
Message-ID: <20250915073933.L7UOtfkj@linutronix.de>
References: <20250904142526.1845999-1-bigeasy@linutronix.de>
 <20250905101502.6525-1-hdanton@sina.com>
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In-Reply-To: <20250905101502.6525-1-hdanton@sina.com>

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.

The difference is that !RT requires two CPU for this to happen while RT
is efficient and can trigger this with just one CPU.

Sebastian