linux-kernel - Re: schedule() && prev/current (Was: [PATCH 3/3] Make get_current() __attribute_

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Date:	Wed, 19 May 2010 12:27:43 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	David Howells <dhowells@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Yong Zhang <yong.zhang0@...il.com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: schedule() && prev/current (Was: [PATCH 3/3] Make
	get_current() __attribute__((const)))

On 05/19, Peter Zijlstra wrote:
>
> On Tue, 2010-05-18 at 23:22 +0200, Oleg Nesterov wrote:
>
> > And, looking at this patch I think that schedule() can be simplified
> > a little bit.
> >
> > "sched: Reassign prev and switch_count when reacquire_kernel_lock() fail"
> > commit 6d558c3ac9b6508d26fd5cadccce51fc9d726b1c says:
> >
> > 	Assume A->B schedule is processing,
> > 	...
> > 	Then on B's context,
> > 	...
> > 	prev and switch_count are related to A
> >
> > How so? switch_count - yes, we should change it. But prev must be
> > equal to B, and it must be equal to current. When we return from
> > switch_to() registers/stack should be restored correctly, so we
> > can do
>
> What if schedule() got called at a different stack depth than we are
> now?
>
> I don't think we can assume anything about the stack context we just
> switched to.

Not sure I understand...

OK. Firstly, we shouldn't worry about the freshly forked tasks, they
never "return" from switch_to() but call ret_from_fork()->schedule_tail(),
right?

Now suppose that A calls schedule() and we switch to B. When switch_to()
returns on B's context, this context (register/stack) matches the previous
context which was used by B when it in turn called schedule(), correct?

IOW. B calls schedule, prev == B. schedule() picks another task, prev
is saved on B's stck after switch_to(). A calls schedule(), prev == A
before context_switch(A, B), but after that switch_to() switches to
B's stack and prev == B.

No?

I am looking into the git history now... and I guess I understand why
reacquire_kernel_lock() uses current. Because schedule() did something
like

	prev = context_switch(prev, next);	// prev == last

	finish_task_switch(prev);

	reacquire_kernel_lock(current);		// prev != current

Oleg.

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