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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>,
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Subject: Re: [patch 00/13] preempt: Make preempt count unconditional
Date: Tue, 15 Sep 2020 10:35:20 -0700
Message-ID: <CAHk-=wht7kAeyR5xEW2ORj7m0hibVxZ3t+2ie8vNHLQfdbN2_g@mail.gmail.com> (raw)
In-Reply-To: <87bli75t7v.fsf@nanos.tec.linutronix.de>

On Tue, Sep 15, 2020 at 1:39 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> OTOH, having a working 'preemptible()' or maybe better named
> 'can_schedule()' check makes tons of sense to make decisions about
> allocation modes or other things.

No. I think that those kinds of decisions about actual behavior are
always simply fundamentally wrong.

Note that this is very different from having warnings about invalid
use. THAT is correct. It may not warn in all configurations, but that
doesn't matter: what matters is that it warns in common enough
configurations that developers will catch it.

So having a warning in "might_sleep()" that doesn't always trigger,
because you have a limited configuration that can't even detect the
situation, that's fine and dandy and intentional.

But having code like

       if (can_schedule())
           .. do something different ..

is fundamentally complete and utter garbage.

It's one thing if you test for "am I in hardware interrupt context".
Those tests aren't great either, but at least they make sense.

But a driver - or some library routine - making a difference based on
some nebulous "can I schedule" is fundamentally and basically WRONG.

If some code changes behavior, it needs to be explicit to the *caller*
of that code.

So this is why GFP_ATOMIC is fine, but "if (!can_schedule())
do_something_atomic()" is pure shite.

And I am not IN THE LEAST interested in trying to help people doing
pure shite. We need to fix them. Like the crypto code is getting
fixed.

                   Linus
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  parent reply index

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
     [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 [this message]
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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