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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Juri Lelli <juri.lelli@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
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	Brian Cain <bcain@codeaurora.org>,
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Subject: Re: [patch 00/13] preempt: Make preempt count unconditional
Date: Mon, 14 Sep 2020 23:39:14 -0700
Message-ID: <CAHk-=wir6LZ=4gHt8VDdASv=TmEMjEUONuzbt=s+DyXPCvxaBA@mail.gmail.com> (raw)
In-Reply-To: <20200915062253.GA26275@gondor.apana.org.au>

On Mon, Sep 14, 2020 at 11:24 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> On Tue, Sep 15, 2020 at 09:20:59AM +0300, Ard Biesheuvel wrote:
> >
> > The documentation of kmap_atomic() states the following:
> >
> >  * The use of kmap_atomic/kunmap_atomic is discouraged - kmap/kunmap
> >  * gives a more generic (and caching) interface. But kmap_atomic can
> >  * be used in IRQ contexts, so in some (very limited) cases we need
> >  * it.
> >
> > so if this is no longer accurate, perhaps we should fix it?
>
> This hasn't been accurate for at least ten years :)

Yeah, that used to be true a long long time ago, but the comment is very stale.

> > 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.

So it's mostly a debug thing.

I say "mostly", because there might be small other details too, like
shared code, and perhaps even a couple of users out in the wild that
depend on the pagefault_disable() inherent in the current
kmap_atomic(), who knows..

So no, the preemption disabling isn't inherent in the operation
itself. But it does have some argument for it.

                   Linus
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Thread overview: 50+ 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
2020-09-14 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
2020-09-14 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
2020-09-14 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 [this message]
     [not found]               ` <87een35woz.fsf@nanos.tec.linutronix.de>
2020-09-15 17:29                 ` Linus Torvalds
     [not found]       ` <87bli75t7v.fsf@nanos.tec.linutronix.de>
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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