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

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Message-ID: <a2ebde260705161617u260bcb55i8c4a56e4a4875954@mail.gmail.com>
Date:	Thu, 17 May 2007 07:17:36 +0800
From:	"Dong Feng" <middle.fengdong@...il.com>
To:	"Phillip Susi" <psusi@....rr.com>
Cc:	"pradeep singh" <2500.pradeep@...il.com>,
	"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?

OK. I think the gap between you and me is the definition of term
*context*. If you go to Linux Kernel Development, 2nd Edition (ISBN
0-672-32720-1), Page 6, then you will read the following:

....  in Linux, ... each processor is doing one of three things at any
given moment:

1. In kernel-space, in process context, ...
2. In kernel-space, in interrupt context, not associated with a process, ...
3. In user-space ...

This list is inclusive. ...

Maybe you prefer other terminology system, but I do like the above
definition given by Robert Love. So maybe in your system *context*
mean something at hardware level and you say ISR is in process
context, but I think it is more like a logical level and agree with
Rovert's definition.

And in hardware level, Robert's *context* definition also mean
something specific, that I started to be aware of. That is, *in the
same context* means a kernel-code is triggered by a user-space code.
*in different context* means a kernel-code is triggered by an external
interrupt source other than a user-space code.

Context has nothing to do with whether an ISR borrow any data
structure of a process, instead, its something logical or related to
causality.

2007/5/16, Phillip Susi <psusi@....rr.com>:
> Dong Feng wrote:
> > If what you say were true, then an ISR would be running in the same
> > context as the interrupted process.
>
> Yes, and it is, as others have said in this thread, which is a good
> reason why ISRs can't sleep.
>
> > But please check any article or
> > book, it will say ISR running in different context from any process.
> > So ISR is considered in its own context, although it shares a lot of
> > things with the interrupted process. I would only say *context* is a
> > higher-level logical concept.
>
> Depends on which book or article you are reading I suppose.  The
> generally accepted and often used thought is that ISRs technically are
> running in the context of the interrupted process, but because that
> context is unknown and therefore should not be used, it is often said
> that they run in no context, or outside of any context.  Sometimes
> people then assume that because they run outside of any ( particular )
> process context, they must be in their own context, but this is a mistake.
>
>
>
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