linux-kernel - Re: [PATCH] iio: trigger: sysfs: Disable irqs before calling iio_trigger

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Message-ID: <a59d204e-aeb0-2649-5e6f-f07815713d1a@metafoo.de>
Date:   Mon, 3 Aug 2020 08:52:54 +0200
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     Christian Eggers <ceggers@...i.de>
Cc:     Jonathan Cameron <jic23@...nel.org>, stable@...r.kernel.org,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: trigger: sysfs: Disable irqs before calling
 iio_trigger_poll()

On 8/3/20 8:44 AM, Christian Eggers wrote:
> Hi Lars,
>
> On Monday, 3 August 2020, 08:37:43 CEST, Lars-Peter Clausen wrote:
>> The sysfs IIO trigger uses irq_work to schedule the iio_trigger_poll()
>> and the promise of irq_work is that the callback will run in hard IRQ
>> context. That's the whole point of it.
>>
>> irq_work_run_list(), which shows up in your callgraph, has as
>> BUG_ON(!irqs_disabled())[1], so we should never even get to calling
>> iio_trigger_poll() if IRQs where not disabled at this point. That's the
>> same condition that triggers the WARN_ON() in __handle_irq_event_percpu.
> is my patch sufficient, or would you prefer a different solution?
The code in normal upstream is correct, there is no need to patch it 
since iio_sysfs_trigger_work() always runs with IRQs disabled.
>
>> Are you using a non-upstream kernel? Maybe a RT kernel?
> I use v5.4.<almost-latest>-rt

That explains it. Have a look at 
0200-irqwork-push-most-work-into-softirq-context.patch.

The right fix for this issue is to add the following snippet to the RT 
patchset.

diff --git a/drivers/iio/trigger/iio-trig-sysfs.c 
b/drivers/iio/trigger/iio-trig-sysfs.c
--- a/drivers/iio/trigger/iio-trig-sysfs.c
+++ b/drivers/iio/trigger/iio-trig-sysfs.c
@@ -161,6 +161,7 @@ static int iio_sysfs_trigger_probe(int id)
      iio_trigger_set_drvdata(t->trig, t);

      init_irq_work(&t->work, iio_sysfs_trigger_work);
+    t->work.flags = IRQ_WORK_HARD_IRQ;

      ret = iio_trigger_register(t->trig);
      if (ret)