linux-kernel - Re: [PATCH 3/4] media: dw100: Fix kernel oops with PREEMPT

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Message-ID: <20260106004928.GL10026@pendragon.ideasonboard.com>
Date: Tue, 6 Jan 2026 02:49:28 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Nicolas Dufresne <nicolas@...fresne.ca>,
	Stefan Klug <stefan.klug@...asonboard.com>,
	Xavier Roumegue <xavier.roumegue@....nxp.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <clrkwllms@...nel.org>, linux-media@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev
Subject: Re: [PATCH 3/4] media: dw100: Fix kernel oops with PREEMPT_RT enabled

On Mon, Jan 05, 2026 at 07:39:33PM -0500, Steven Rostedt wrote:
> On Tue, 6 Jan 2026 01:59:21 +0200 Laurent Pinchart wrote:
> 
> > > That's interesting, do you plan to update more drivers ? There is a lot of m2m
> > > using hard IRQ to minimize the idle time (save a context switch), but RT support
> > > might be more worth then that.  
> > 
> > This is a part of PREEMPT_RT that puzzles me. By turning regular
> > spinlocks into mutexes, RT seems to break drivers that use those
> > spinlocks in hard IRQ handlers. That's a very large number of drivers
> > given how widespread regular spinlock usage is. Do drivers need to be
> > manually converted to either raw spinlocks or threaded IRQ handlers ?
> 
> No. Pretty much all interrupts are converted into threaded interrupt
> handlers unless IRQF_NO_THREAD, IRQF_PERCPU, or IRQF_ONESHOT are specified.
> 
> The interrupt line is disabled until the thread handler is called.
> 
> > What about non-RT kernels, how can a driver avoid the thread scheduling
> > penalty in those cases, do they need to manually select between
> > request_irq() and request_threaded_irq() based on if RT is enabled ?
> > This puzzles me, it feels like I must be missing something.
> 
> The issue here is that the interrupt handler specifies ONESHOT which causes
> the handler to be executed in hard interrupt context.

Gotcha.

Stefan, please explain in the commit message why the ONESHOT flag is
set by the driver.

-- 
Regards,

Laurent Pinchart