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

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Message-ID: <20260105193933.40485807@gandalf.local.home>
Date: Mon, 5 Jan 2026 19:39:33 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
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 Tue, 6 Jan 2026 01:59:21 +0200
Laurent Pinchart <laurent.pinchart@...asonboard.com> 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.

-- Steve