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SubjectRe: 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@redhat.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@gmail.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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