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Message-ID: <CA+55aFy2Sno+bS0A2k0cMWpEJy-bpXufSAw3+ufrfQYbp9rcMQ@mail.gmail.com>
Date: Mon, 11 Jul 2016 09:06:58 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: linux-arch@...r.kernel.org, Nadav Amit <nadav.amit@...il.com>,
Mark Rutland <mark.rutland@....com>, Kees Cook <keescook@...omium.org>,
Borislav Petkov <bp@...en8.de>, Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
Jann Horn <jann@...jh.net>, Heiko Carstens <heiko.carstens@...ibm.com>,
Brian Gerst <brgerst@...il.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 26/29] sched: Allow putting
thread_info into task_struct
On Jul 11, 2016 7:55 AM, "Andy Lutomirski" <luto@...capital.net> wrote:
>
> How do you intend to find 'current' to get to the preempt count
> without first disabling preemption?
Actually, that is the classic case of "not a problem".
The thing is, it doesn't matter if you schedule away while looking up
current or the preempt count - because both values are idempotent wet
scheduling.
So until you do the wire that actually disables preemption you can schedule
away as much as you want, and after that write you no longer will.
This is different wrt a per-cpu area - which is clearly not idempotent wrt
scheduling.
The reason per-cpu works on x86 is that we have an atomic rmw operation
that is *also* atomic wrt the CPU lookup (thanks to the segment base)
Linus
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