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From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	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>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Ben Segall <bsegall@google.com>, Linux-MM <linux-mm@kvack.org>,
	linux-hexagon@vger.kernel.org, Will Deacon <will@kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Anton Ivanov <anton.ivanov@cambridgegreys.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	Brian Cain <bcain@codeaurora.org>,
	Richard Weinberger <richard@nod.at>,
	Russell King <linux@armlinux.org.uk>,
	Ard Biesheuvel <ardb@kernel.org>, David Airlie <airlied@linux.ie>,
	Ingo Molnar <mingo@redhat.com>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Mel Gorman <mgorman@suse.de>,
	intel-gfx <intel-gfx@lists.freedesktop.org>,
	Matt Turner <mattst88@gmail.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	linux-xtensa@linux-xtensa.org, Shuah Khan <shuah@kernel.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Jeff Dike <jdike@addtoit.com>,
	linux-um <linux-um@lists.infradead.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	rcu@vger.kernel.org, linux-m68k <linux-m68k@lists.linux-m68k.org>,
	Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Richard Henderson <rth@twiddle.net>,
	Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	alpha <linux-alpha@vger.kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [Intel-gfx] [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


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Intel-gfx mailing list
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  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 [Intel-gfx] [patch 00/13] preempt: Make preempt count unconditional Thomas Gleixner
2020-09-14 20:42 ` [Intel-gfx] [patch 01/13] lib/debug: Remove pointless ARCH_NO_PREEMPT dependencies Thomas Gleixner
2020-09-14 20:42 ` [Intel-gfx] [patch 02/13] preempt: Make preempt count unconditional Thomas Gleixner
2020-09-14 20:42 ` [Intel-gfx] [patch 03/13] preempt: Clenaup PREEMPT_COUNT leftovers Thomas Gleixner
2020-09-16 10:56   ` Valentin Schneider
2020-09-14 20:42 ` [Intel-gfx] [patch 04/13] lockdep: " Thomas Gleixner
2020-09-15 16:11   ` Will Deacon
2020-09-14 20:42 ` [Intel-gfx] [patch 05/13] mm/pagemap: " Thomas Gleixner
2020-09-14 20:42 ` [Intel-gfx] [patch 06/13] locking/bitspinlock: " Thomas Gleixner
2020-09-15 16:10   ` Will Deacon
2020-09-14 20:42 ` [Intel-gfx] [patch 07/13] uaccess: " Thomas Gleixner
2020-09-14 20:42 ` [Intel-gfx] [patch 08/13] sched: " Thomas Gleixner
2020-09-16 10:56   ` Valentin Schneider
2020-09-14 20:42 ` [Intel-gfx] [patch 09/13] ARM: " Thomas Gleixner
2020-09-14 20:42 ` [Intel-gfx] [patch 10/13] xtensa: " Thomas Gleixner
2020-09-14 20:42 ` [Intel-gfx] [patch 11/13] drm/i915: " Thomas Gleixner
2020-09-14 20:42 ` [Intel-gfx] [patch 12/13] rcutorture: " Thomas Gleixner
2020-09-14 20:42 ` [Intel-gfx] [patch 13/13] preempt: Remove PREEMPT_COUNT from Kconfig Thomas Gleixner
2020-09-14 20:54 ` [Intel-gfx] [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         ` [Intel-gfx] [PATCH] crypto: lib/chacha20poly1305 - Set SG_MITER_ATOMIC unconditionally Herbert Xu
2020-09-15  6:20         ` [Intel-gfx] [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
2020-09-14 22:01 ` [Intel-gfx] ✗ Fi.CI.BUILD: failure for " Patchwork

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