Re: Kernelnewbies Digest, Vol 44, Issue 51

Vishwas Srivastava <vishu.kernel@xxxxxxxxx> · Mon, 28 Jul 2014 22:58:59 +0530

Hi Jeff,           yes, your guess is correct. The place i checked  in_interrupt() was a region before which a call to spin_lock_bh() was made.
So what is the rule?
if in_interrupt() return me true then that is the region of the code which can not be context switched or if i am out of interrupt (in kernel thread) and have called spin_lock_bh(), then this macro is not reliable to safely find out if the region below this can not be context switched?
If that is the case, then perhaps only the "false" return of the macro in_interrupt is reliable.
thanks,
Vishwas S

On Mon, Jul 28, 2014 at 9:54 PM, Jeff Haran <Jeff.Haran@xxxxxxxxxx> wrote:

From:
 kernelnewbies-bounces@xxxxxxxxxxxxxxxxx [mailto:kernelnewbies-bounces@xxxxxxxxxxxxxxxxx]
On Behalf Of Vishwas Srivastava

Sent: Saturday, July 26, 2014 10:07 AM

To: kernelnewbies@xxxxxxxxxxxxxxxxx

Subject: Re: Kernelnewbies Digest, Vol 44, Issue 51

Hi All,

          I have a doubt regarding the  threaded interrupt handing mechanism.

what is the kernel context of execution while executing interrupt thread.

if i check for macro

in_irq() or in_interrupt() withing handler thread , my understanding is that they both are going to return me 0 (since we are in kernel thread),

as threads runs in process context.

Also, since they are "kernel threads", they must be schedulable enteties.

what conceptually deviated me from my understanding is one of the driver which had a threaded interrupt mechanism and when i checked

in_interrupt() macro, it returned me a non zero value so i am a bit confused.

By any chance had you disabled interrupts or bottom halves when you made
 this call to in_interrupt()?
The reason I ask is the last time I looked into this the conclusion I came
 to was that in_interrupt() would return non-zero under a couple of different conditions; 1) the code was really executing in either a bottom half or IRQ context, or; 2) the code was executing in process context but had previously
 disabled interrupts or bottom halves, e.g. a call to spin_lock_bh() or anything else that disables bottom halves. At the time I was trying to figure out how to tell for sure whether or not a given function was being called from
 a bottom half (as opposed to executing in process context with bottom halves disabled) and from what I could see at the time, there was no way to distinguish the two conditions. Disclaimer: I’m no expert on this stuff, just reporting what I concluded at the
 time from eyeballing the code. Perhaps I missed something.

Jeff Haran

thanks in advance.

Vishwas S

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