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

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Message-ID: <20250918154937.RQqkeYxI@linutronix.de>
Date: Thu, 18 Sep 2025 17:49:37 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Hillf Danton <hdanton@...a.com>
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 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?

> > The difference is that !RT requires two CPU for this to happen while RT
> > is efficient and can trigger this with just one CPU.
> 
> In case of RT OTOH, false positive deadlock could be triggered because
> canceling taskletB has nothing to do with the callback of taskletC.
> 
> In short I am highlighting the gap between per-timer/tasklet and per-cpu.

I don't see a problem here.

Sebastian