From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Herbert Xu <herbert@gondor.apana.org.au>
Cc: Ard Biesheuvel <ardb@kernel.org>,
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Shuah Khan <shuah@kernel.org>,
rcu@vger.kernel.org
Subject: Re: [patch 00/13] preempt: Make preempt count unconditional
Date: Tue, 15 Sep 2020 09:24:44 +0200 [thread overview]
Message-ID: <87een35woz.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <CAHk-=wir6LZ=4gHt8VDdASv=TmEMjEUONuzbt=s+DyXPCvxaBA@mail.gmail.com>
On Mon, Sep 14 2020 at 23:39, Linus Torvalds wrote:
> On Mon, Sep 14, 2020 at 11:24 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>> > But another reason I tried to avoid kmap_atomic() is that it disables
>> > preemption unconditionally, even on 64-bit architectures where HIGHMEM
>> > is irrelevant. So using kmap_atomic() here means that the bulk of
>> > WireGuard packet encryption runs with preemption disabled, essentially
>> > for legacy reasons.
>>
>> Agreed. We should definitely fix that.
>
> Well, honestly, one big reason for that is debugging.
>
> The *semantics* of the kmap_atomic() is in the name - you can't sleep
> in between it and the kunmap_atomic().
>
> On any sane architecture, kmap_atomic() ends up being a no-op from an
> implementation standpoint, and sleeping would work just fine.
>
> But we very much want to make sure that people don't then write code
> that doesn't work on the bad old 32-bit machines where it really needs
> that sequence to be safe from preemption.
Alternatively we just make highmem a bit more expensive by making these
maps preemptible. RT is doing this for a long time and it's not that
horrible.
The approach is to keep track about the number of active maps in a task
and on an eventual context switch save them away in the task struct and
restore them when the task is scheduled back in.
Thanks,
tglx
next prev parent reply other threads:[~2020-09-15 7:24 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-14 20:42 Thomas Gleixner
2020-09-14 20:42 ` [patch 01/13] lib/debug: Remove pointless ARCH_NO_PREEMPT dependencies Thomas Gleixner
2020-09-14 20:42 ` [patch 02/13] preempt: Make preempt count unconditional Thomas Gleixner
2020-09-14 20:42 ` [patch 03/13] preempt: Clenaup PREEMPT_COUNT leftovers Thomas Gleixner
2020-09-16 10:56 ` Valentin Schneider
2020-09-14 20:42 ` [patch 04/13] lockdep: " Thomas Gleixner
2020-09-15 16:11 ` Will Deacon
2020-09-14 20:42 ` [patch 05/13] mm/pagemap: " Thomas Gleixner
2020-09-15 0:52 ` kernel test robot
2020-09-14 20:42 ` [patch 06/13] locking/bitspinlock: " Thomas Gleixner
2020-09-15 16:10 ` Will Deacon
2020-09-14 20:42 ` [patch 07/13] uaccess: " Thomas Gleixner
2020-09-14 20:42 ` [patch 08/13] sched: " Thomas Gleixner
2020-09-16 10:56 ` Valentin Schneider
2020-09-14 20:42 ` [patch 09/13] ARM: " Thomas Gleixner
2020-09-14 20:42 ` [patch 10/13] xtensa: " Thomas Gleixner
2020-09-14 20:42 ` [patch 11/13] drm/i915: " Thomas Gleixner
2020-09-14 20:42 ` [patch 12/13] rcutorture: " Thomas Gleixner
2020-09-14 20:42 ` [patch 13/13] preempt: Remove PREEMPT_COUNT from Kconfig Thomas Gleixner
2020-09-14 20:54 ` [patch 00/13] preempt: Make preempt count unconditional Steven Rostedt
2020-09-14 20:59 ` Linus Torvalds
2020-09-14 21:55 ` Thomas Gleixner
2020-09-14 22:24 ` Linus Torvalds
2020-09-14 22:37 ` Linus Torvalds
2020-09-15 3:21 ` [PATCH] crypto: lib/chacha20poly1305 - Set SG_MITER_ATOMIC unconditionally Herbert Xu
2020-09-15 6:20 ` [patch 00/13] preempt: Make preempt count unconditional Ard Biesheuvel
2020-09-15 6:22 ` Herbert Xu
2020-09-15 6:39 ` Linus Torvalds
2020-09-15 7:24 ` Thomas Gleixner [this message]
2020-09-15 17:29 ` Linus Torvalds
2020-09-15 8:39 ` Thomas Gleixner
2020-09-15 17:35 ` Linus Torvalds
2020-09-15 19:57 ` Thomas Gleixner
2020-09-16 18:34 ` Linus Torvalds
2020-09-16 7:37 ` Daniel Vetter
2020-09-16 15:29 ` Paul E. McKenney
2020-09-16 18:32 ` Linus Torvalds
2020-09-16 20:43 ` Paul E. McKenney
2020-09-17 6:38 ` Ard Biesheuvel
2020-09-16 20:29 ` Daniel Vetter
2020-09-16 20:58 ` Paul E. McKenney
2020-09-16 21:43 ` Daniel Vetter
2020-09-16 22:39 ` Paul E. McKenney
2020-09-17 7:52 ` Daniel Vetter
2020-09-17 16:28 ` Paul E. McKenney
2020-09-29 8:19 ` Michal Hocko
2020-09-29 8:19 ` Michal Hocko
2020-09-29 8:20 ` Michal Hocko
2020-09-29 8:21 ` Michal Hocko
2020-09-29 8:23 ` Michal Hocko
2020-09-29 9:00 ` Daniel Vetter
2020-09-29 14:54 ` Michal Hocko
2020-09-16 19:23 ` Matthew Wilcox
2020-09-16 20:48 ` Paul E. McKenney
2020-09-15 17:25 ` Paul E. McKenney
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