linux-kernel - Re: [PATCH] Video : Amba: Use in_interrupt() in clcdfb

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Message-ID: <20120311150311.GC13336@n2100.arm.linux.org.uk>
Date:	Sun, 11 Mar 2012 15:03:11 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	santosh prasad nayak <santoshprasadnayak@...il.com>
Cc:	FlorianSchandinat@....de, linux-fbdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] Video : Amba: Use in_interrupt() in clcdfb_sleep().

On Sun, Mar 11, 2012 at 08:18:32PM +0530, santosh prasad nayak wrote:
> On Sun, Mar 11, 2012 at 8:06 PM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > On Sun, Mar 11, 2012 at 07:47:27PM +0530, santosh prasad nayak wrote:
> >> Not to use in_atomic()  in driver code.
> >>
> >>  Following article  inspired me to do the change.
> >> http://lwn.net/Articles/274695/
> >>
> >> "in_atomic() is for core kernel use only. Because in special
> >> circumstances (ie: kmap_atomic()) we run inc_preempt_count() even on
> >> non-preemptible kernels to tell the per-arch fault handler that it was
> >> invoked by copy_*_user() inside kmap_atomic(), and it must fail.
> >> In other words, in_atomic() works in a specific low-level situation,
> >> but it was never meant to be used in a wider context. Its placement in
> >> hardirq.h next to macros which can be used elsewhere was, thus, almost
> >> certainly a mistake. As Alan Stern pointed out, the fact that Linux
> >> Device Drivers recommends the use of in_atomic() will not have helped
> >> the situation. Your editor recommends that the authors of that book be
> >> immediately sacked. "
> >>
> >> In the present case, we just check whether its an IRQ context or user
> >> context. So for that
> >> we can use "in_interrupt()".
> >>
> >> Greg also mentions the same in the following mail.
> >> http://www.spinics.net/lists/newbies/msg43402.html
> >
> > In which case, we'll just have to do mdelay() and forget about allowing
> > anything else to run for the 20ms that we need to sleep.  Sucky but
> > that's the way things are.
> 
>  mdelay() or msleep() are there before. I did not change that.
> 
> 
> my point is :  in_atomic()  vs "in_interrupt()".
> We should avoid to use "in_atomic()" in driver code.
> 
> In the present case to check IRQ context "in_interrupt()" should be preferred.

in_interrupt() won't tell us if we're being called with spinlocks held,
which _is_ a possibility because this can be called from printk(), for
oops dumps and the like.

in_interrupt() just means that we're inside a hard or soft interrupt,
or nmi.  It says nothing about whether msleep() is possible.
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