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

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Date:	Sun, 11 Mar 2012 20:49:27 +0530
From:	santosh prasad nayak <santoshprasadnayak@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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 8:33 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> 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.


in_atomic() is also not  error free.  I found following comment in
include/linux/hardirq.h.  How do you handle it in non-preemptible
kernel ?

/*
 * Are we running in atomic context?  WARNING: this macro cannot
 * always detect atomic context; in particular, it cannot know about
 * held spinlocks in non-preemptible kernels.  Thus it should not be
 * used in the general case to determine whether sleeping is possible.
 * Do not use in_atomic() in driver code.
 */
#define in_atomic()     ((preempt_count() & ~PREEMPT_ACTIVE) != 0)



regards
Santosh
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