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From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
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Subject: Re: [patch 00/13] preempt: Make preempt count unconditional
Date: Mon, 14 Sep 2020 23:55:24 +0200
Message-ID: <871rj4owfn.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <CAHk-=win80rdof8Pb=5k6gT9j_v+hz-TQzKPVastZDvBe9RimQ@mail.gmail.com>

On Mon, Sep 14 2020 at 13:59, Linus Torvalds wrote:
> On Mon, Sep 14, 2020 at 1:45 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>>
>> Recently merged code does:
>>
>>          gfp = preemptible() ? GFP_KERNEL : GFP_ATOMIC;
>>
>> Looks obviously correct, except for the fact that preemptible() is
>> unconditionally false for CONFIF_PREEMPT_COUNT=n, i.e. all allocations in
>> that code use GFP_ATOMIC on such kernels.
>
> I don't think this is a good reason to entirely get rid of the
> no-preempt thing.

I did not say that this is a good reason. It just illustrates the
problem.

> The above is just garbage. It's bogus. You can't do it.
>
> Blaming the no-preempt code for this bug is extremely unfair, imho.

I'm not blaming the no-preempt code. I'm blaming inconsistency and there
is no real good argument for inconsistent behaviour, TBH.

> And the no-preempt code does help make for much better code generation
> for simple spinlocks.

Yes it does generate better code, but I tried hard to spot a difference
in various metrics exposed by perf. It's all in the noise and I only
can spot a difference when the actual preemption check after the
decrement which still depends on CONFIG_PREEMPT is in place, but that's
not the case for PREEMPT_NONE or PREEMPT_VOLUNTARY kernels where the
decrement is just a decrement w/o any conditional behind it.

> Where is that horribly buggy recent code? It's not in that exact
> format, certainly, since 'grep' doesn't find it.

Bah, that was stuff in next which got dropped again.

But just look at any check which uses preemptible(), especially those
which check !preemptible():

In the X86 #GP handler we have:

	/*
	 * To be potentially processing a kprobe fault and to trust the result
	 * from kprobe_running(), we have to be non-preemptible.
	 */
	if (!preemptible() &&
	    kprobe_running() &&
	    kprobe_fault_handler(regs, X86_TRAP_GP))
		goto exit;

and a similar check in the S390 code in kprobe_exceptions_notify(). That
all magically 'works' because that code might have been actually tested
with lockdep enabled which enforces PREEMPT_COUNT...

The SG code has some interesting usage as well:

		if (miter->__flags & SG_MITER_ATOMIC) {
			WARN_ON_ONCE(preemptible());
			kunmap_atomic(miter->addr);

How is that WARN_ON_ONCE() supposed to catch anything? Especially as
calling code does:

	flags = SG_MITER_TO_SG;
	if (!preemptible())
		flags |= SG_MITER_ATOMIC;

which is equally useless on kernels which have PREEMPT_COUNT=n.

There are bugs which are related to in_atomic() or other in_***() usage
all over the place as well.

Inconsistency at the core level is a clear recipe for disaster and at
some point we have to bite the bullet and accept that consistency is
more important than the non measurable 3 cycles?

Thanks,

        tglx
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  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 [this message]
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
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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