linux-kernel - Re: [PATCH 1/2] bdisp: Fix a possible sleep-in-atomic bug in bdisp_hw

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Date:   Sat, 16 Dec 2017 12:14:27 -0200
From:   Mauro Carvalho Chehab <mchehab@...nel.org>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     Fabien DESSENNE <fabien.dessenne@...com>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Benjamin GAIGNARD <benjamin.gaignard@...com>,
        Hans Verkuil <hverkuil@...all.nl>
Subject: Re: [PATCH 1/2] bdisp: Fix a possible sleep-in-atomic bug in
 bdisp_hw_reset

Em Sat, 16 Dec 2017 19:53:55 +0800
Jia-Ju Bai <baijiaju1990@...il.com> escreveu:

> Hi,
> 
> On 2017/12/15 22:51, Fabien DESSENNE wrote:
> > Hi
> >
> > On 12/12/17 14:47, Jia-Ju Bai wrote:  
> >> The driver may sleep under a spinlock.
> >> The function call path is:
> >> bdisp_device_run (acquire the spinlock)
> >>     bdisp_hw_reset
> >>       msleep --> may sleep
> >>
> >> To fix it, msleep is replaced with mdelay.  
> > May I suggest you to use readl_poll_timeout_atomic (instead of the whole
> > "for" block): this fixes the problem and simplifies the code?  
> 
> Okay, I have submitted a patch according to your advice.
> You can have a look :)

This can still be usind mdelay() to wait for a long time.

It doesn't seem wise to do that, as it could cause system
contention. Couldn't this be reworked in a way to avoid
having the spin locked while sleeping?

Once we had a similar issue on Siano, and it was solved by this

commit 3cdadc50bbe8f04c1231c8af614cafd7ddd622bf
Author: Richard Zidlicky <rz@...ux-m68k.org>
Date:   Tue Aug 24 09:52:36 2010 -0300

    V4L/DVB: dvb: fix smscore_getbuffer() logic
    
    Drivers shouldn't sleep while holding a spinlock. A previous workaround
    were to release the spinlock before callinc schedule().
    
    This patch uses a different approach: it just waits for the
    siano hardware to answer.
    
    Signed-off-by: Richard Zidlicky <rz@...ux-m68k.org>
    Cc: stable@...nel.org
    Signed-off-by: Mauro Carvalho Chehab <mchehab@...hat.com>

The code as changed to use wait_event() at the kthread that was
waiting for data to arrive. Only when the data is ready, the
code with the spin lock is called.

It made the driver a way more stable, and didn't add any penalties
of needing to do long delays on a non-interruptible code.

Thanks,
Mauro