Re: [PATCH 2.5.73] Signal stack fixes #1 introduce PF_SS_ACTIVE

[Linus Torvalds]

On Fri, 4 Jul 2003, Jörn Engel wrote:
> 
> This is the generic part of the signal stack fixes, it simply
> introduces a new PF-flag that indicates whether we are using the
> signal stack right now or not.

My reason for disliking this patch is that it adds user-space information 
to the kernel - in a place where user space cannot get at it.

In particular, any traditional cooperative user-space threading package
wants to switch its own stack around, and they all do it by just changing
%esp directly. The whole point of such threading is that it's _fast_,
since it doesn't need any kernel support (and since it's cooperative, you
can avoid locking).

The old "optimization" that you didn't like was not an optimization at 
all: it got the case of user space changing stacks _right_, while still 
allowing yet another stack for signal handling and exiting the signal by 
hand.

Does anybody do that? I don't know. But it was done the way it was done on 
purpose.

		Linus