linux-kernel - Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

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Date:	Sat, 25 Jun 2016 18:23:19 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>, Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core)

On Sat, Jun 25, 2016 at 4:30 PM, Andy Lutomirski <luto@...capital.net> wrote:
>
> Maybe I'm misunderstanding the role of release_task.  It looks like
> there's this path in the scheduler I can borrow:
>
>     if (unlikely(prev_state == TASK_DEAD)) {
>
> With a kludge in place to free the stack in there and release_task and
> __put_task_struct, whichever is first, I get a nice speedup.
> Benchmarks coming later on.  Can I rely on that code path always being
> called?

Absolutely. That's the normal "task is done, put the thread struct".
IOW, that's the final "put_task_struct()" that the task "itself" calls
as it exits - there may be other things that hold a reference to the
task struct, but that's where you should free the stack because the
thread itself is done with it..

            Linus