linux-kernel - Re: Can current macro be accessed from interrupt context?

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Date:	Thu, 22 Oct 2009 04:43:38 -0700
From:	"Leonidas ." <leonidas137@...il.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Can current macro be accessed from interrupt context?

On Thu, Oct 22, 2009 at 4:27 AM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> On Thu, Oct 22, 2009 at 04:10:49AM -0700, Leonidas . wrote:
>> Hello,
>>
>> Going through UTLK, it says that current macro makes sense only in
>> case of process context,
>> in case of interrupt context it is invalid.
>
>
> Indeed, usually it makes only sense in process context.
>
>
>> But current would still be pointing to interrupted process right? The
>> pointer would still be valid?
>
>
>
> Yeah, unless you irq handler executes in a threaded interrupt, in
> which case current will be pointing to the given irq thread.

Threaded interrupt handlers? I am sorry, I am new to kernel development
and not fully aware of the intricacies. Do you mean bottom half handlers here?

> There are few tiny cases where it is unsafe to deref "current",
> such as the very beginning of a cpu's awakening, when the per cpu
> datas are not yet ready for this cpu.
>

Okay, I guess, in my case this would not be a problem then since this is about
early init.

>> Can I safely assume that whether or not interrupt handlers are
>> executing on separate stacks or
>> interrupted threads stack, current macro can be accessed from interrupt context?
>
>
> Yep.
> For example we do that in the function graph tracer. Because we store return
> addresses of functions in the "current" task structure. Even if the task
> is interrupted, it still makes sense to use current because we want
> to know the flow of execution as a linear thing per cpu, the interrupt
> is part of that flow
>
> I hope that helps.
>

Can you give pointers to the source code of this project?

> Frederic.
>
>


-Leo.
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