linux-kernel - Re: How to check whether executing in atomic context?

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Date:	Wed, 14 Oct 2009 19:39:29 +0200
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	"Leonidas ." <leonidas137@...il.com>
CC:	Gleb Natapov <gleb@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: How to check whether executing in atomic context?

On 10/14/2009 12:24 PM, Leonidas . wrote:
> On Wed, Oct 14, 2009 at 3:13 AM, Gleb Natapov <gleb@...hat.com> wrote:
>> On Wed, Oct 14, 2009 at 02:21:22AM -0700, Leonidas . wrote:
>>> On Tue, Oct 13, 2009 at 11:36 PM, Leonidas . <leonidas137@...il.com> wrote:
>>> > Hi List,
>>> >
>>> > I am working on a profiler kind of module, the exported apis of my module can be
>>> > called from process context and interrupt context as well. Depending on the
>>> > context I am called in, I need to call sleepable/nonsleepable variants
>>> > of my internal bookkeeping functions.
>>> >
>>> > I am aware of in_interrupt() call which can be used to check current
>>> > context and take action accordingly.
>>> >
>>> > Is there any api which can help figure out whether we are executing while hold a spinlock? I.e
>>> > an api which can help figure out sleepable/nonsleepable context? If it is not there, what can
>>> > be done for writing the same? Any pointers will be helpful.
[...]
>>>   While searching through the sources, I found this,
>>>
>>>   97/*
>>>   98 * Are we running in atomic context?  WARNING: this macro cannot
>>>   99 * always detect atomic context; in particular, it cannot know about
>>>  100 * held spinlocks in non-preemptible kernels.  Thus it should not be
>>>  101 * used in the general case to determine whether sleeping is possible.
>>>  102 * Do not use in_atomic() in driver code.
>>>  103 */
>>>  104#define in_atomic()     ((preempt_count() & ~PREEMPT_ACTIVE) != PREEMPT_INATOMIC_BASE)
>>>  105
>>>
>>> this just complicates the matter, right? This does not work in general case but I think this
>>> will always work if the kernel is preemptible.
>>>
>>> Is there no way to write a generic macro?
[...]
>> Attached patch make in_atomic() to work for non-preemptable kernels too.
>> Doesn't look to big or scary.
>>
>> Disclaimer: tested only inside kvm guest 64bit, haven't measured overhead.
[...]
> Unbelievable! I was just thinking about the logic to achieve the same, and
> someone has already done this. Thanks for the patch.

I don't know whether Gleb's patch works or doesn't work as you require
it.  But my opinion is that the recommendation "do not use in_atomic()
in driver code" is valid nevertheless.

Very often the better course of action is to change your API from

void my_routine()
{
	if (in_atomic())
		this;
	else
		that;
}

to either

void my_routine(bool can_sleep)
{
	if (!can_sleep)
		this;
	else
		that;
}

or to

void my_routine_atomic()
{
	this;
}

void my_routine()
{
	that;
}

In other words, let the caller of your routine tell it whether it's
atomic context or not.

Instead of a "bool can_sleep" argument, a "gfp_t flags" argument is
often used.

Or provide only the my_routine_atomic() variant if the difference to the
sleeping variant isn't huge.
-- 
Stefan Richter
-=====-==--= =-=- -===-
http://arcgraph.de/sr/
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