linux-kernel - Re: Why can't we sleep in an ISR?

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Message-ID: <a2ebde260705150028j16cc12a2ya6d5906c97ce193b@mail.gmail.com>
Date:	Tue, 15 May 2007 15:28:06 +0800
From:	"Dong Feng" <middle.fengdong@...il.com>
To:	"pradeep singh" <2500.pradeep@...il.com>
Cc:	"Bahadir Balban" <bahadir.balban@...il.com>,
	"Learning Linux" <learninglinux4@...il.com>,
	kernelnewbies@...linux.org, linux-newbie@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Why can't we sleep in an ISR?

> >
> > I don't think so but I am not sure.
>
> Aliter, i think so.How can an interrupt's execution time go
> unaccounted then?
> I guess it does not, only the current processes running
> time is accounted for.
> Thoughts?
>

The interrupt handler's execution time will definitely defer the
execution of the process, but I think it does not steal the process's
time slice (the time_slice field not subtracted).

> > > Doesn't it run in current process's context ?
> > >
> >
> > No. I think the concept of process context is a higher-level logical
> > concept. Though the interrupt share stack with the interrupted
> > process, in my opinion it logically does not share the context with
> > the process.
>
> Yes, you are right as i can infer. thats why ISRs
> are special kernel control paths.
> But the poster asked, why can't we make ISRs to
> share context with the interrupted process
> if
> it not holding any locks? This is rather a desing issues
> IMO rather than imlementation, isnt it?
>
> I guess even if it is possible, it would over complicate the handler code.
> Better trying to keep it simple i guess. Please CMIIW

My understanding is, the ISR is in different context from the process
because of the definition of term *context*. In my opinion, to say two
code pieces running in the same context means that two pieces of code
has some logical relationship to meet a common objective. That's why I
said *context* is a higher-level logical concept. It's not a concept
defined in the level of hardware or code implementation, but instead
in the level of logical. I think, by its definition, it makes no sense
to say an ISR share context with the process interrupted by it because
an ISR just randomly interrupts a process, with no logical
relationship.
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