From: Thomas Gleixner <tglx@linutronix.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Juri Lelli <juri.lelli@redhat.com>,
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Subject: Re: [patch 00/13] preempt: Make preempt count unconditional
Date: Mon, 14 Sep 2020 23:55:24 +0200 [thread overview]
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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next prev parent reply other threads:[~2020-09-15 7:08 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-14 20:42 [patch 00/13] preempt: Make preempt count unconditional 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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