linux-kernel - Re: [Intel-gfx] [PATCH] drm/i915/fence: Do not use TIMER

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Date:   Thu, 28 Feb 2019 10:19:41 +0000
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [Intel-gfx] [PATCH] drm/i915/fence: Do not use TIMER_IRQSAFE

Quoting Thomas Gleixner (2019-02-28 10:09:26)
> On Thu, 28 Feb 2019, Chris Wilson wrote:
> 
> > Quoting Sebastian Andrzej Siewior (2019-02-26 16:00:38)
> > > On 2019-02-12 17:28:57 [+0100], To linux-kernel@...r.kernel.org wrote:
> > > > The timer is initialized with TIMER_IRQSAFE flag. It does look like the
> > > > timer callback requires this flag at all. Its sole purpose is to ensure
> > > > synchronisation in the workqueue code.
> > > > 
> > > > Remove TIMER_IRQSAFE flag because it is not required.
> > > 
> > > ping
> > 
> > We call del_timer_sync from irq context, which mandates using
> > TIMER_IRQSAFE. Failure to do so results in lots of explosions across CI
> > -- every machine managed to hit the warning.
> > 
> > It may not be the best of api, but it's the only one available for the
> > driver to use...
> 
> The comment in the header files says clearly:
> 
>  * Note: The irq disabled callback execution is a special case for
>  * workqueue locking issues. It's not meant for executing random crap
>  * with interrupts disabled. Abuse is monitored!                                   
> 
> So what's so special in drm that you need to call del_timer_sync() from
> interrupt context?

There's no protection against fence signaling from inside interrupt
context, and a lot of pressure to do so.
-Chris