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Date: Thu, 23 Jun 2016 21:46:17 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Oleg Nesterov <oleg@...hat.com>, Andy Lutomirski <luto@...capital.net>,
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>, Brian Gerst <brgerst@...il.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
Josh Poimboeuf <jpoimboe@...hat.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 Thu, Jun 23, 2016 at 12:34:24PM -0700, Linus Torvalds wrote:
> On Thu, Jun 23, 2016 at 12:11 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Didn't we talk about using SLAB_DESTROY_BY_RCU for task_struct before?
> > If that is possible, a reuse in per-cpu cache is equally possible.
> >
> > All we really want to guarantee is that the memory remains a
> > task_struct, it need not remain the same task, right?
>
> No, we can't do SLAB_DESTROY_BY_RCU for the task_struct itself,
> because the RCU list traversal does expect that the thread and task
> lists are stable even if it walks into a "stale" struct task_struct.
Indeed.
OK, so the situation we talked about before is different, we wanted to
do SLAB_DESTROY_BY_RCU on top of the existing delayed_put_task_struct()
to get a double grace period.
The problem was for things like rq->curr, which isn't RCU managed as
such, we could still do:
rcu_read_lock();
task = rq->curr;
and rely on task being _a_ task_struct, even though it might not be the
self-same task we thought we had.
So yes, not an option and I was stitching together two half remembered
situations to create utter nonsense.
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