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From: Peter Zijlstra <peterz@infradead.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>, Nadav Amit <namit@vmware.com>,
	Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Borislav Petkov <bp@alien8.de>,
	"Woodhouse, David" <dwmw@amazon.co.uk>,
	daniel@iogearbox.net, guro@fb.com
Subject: Re: [RFC PATCH 1/5] x86: introduce preemption disable prefix
Date: Fri, 19 Oct 2018 10:22:02 +0200	[thread overview]
Message-ID: <20181019082202.GB3121@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20181019050051.bs2kk4sagu7hpwxx@ast-mbp.dhcp.thefacebook.com>

On Thu, Oct 18, 2018 at 10:00:53PM -0700, Alexei Starovoitov wrote:
> > 
> > >
> > > Another example is __BPF_PROG_RUN_ARRAY(), which also uses
> > > preempt_enable_no_resched().
> > 
> > Alexei, I think this code is just wrong.
> 
> why 'just wrong' ?

Because you lost a preemption point, this is a no-no.

> 
> > Do you know why it uses
> > preempt_enable_no_resched()?
> 
> dont recall precisely.
> we could be preemptable at the point where macro is called.
> I think the goal of no_resched was to avoid adding scheduling points
> where they didn't exist before just because a prog ran for few nsec.
> May be Daniel or Roman remember.

No, you did the exact opposite, where there previously was a preemption,
you just ate it. The band saw didn't get stopped in time, you loose your
hand etc..

  reply	other threads:[~2018-10-19  8:22 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-18  0:54 [RFC PATCH 0/5] x86: dynamic indirect call promotion Nadav Amit
2018-10-18  0:54 ` [RFC PATCH 1/5] x86: introduce preemption disable prefix Nadav Amit
2018-10-18  1:22   ` Andy Lutomirski
2018-10-18  3:12     ` Nadav Amit
2018-10-18  3:26       ` Nadav Amit
2018-10-18  3:51       ` Andy Lutomirski
2018-10-18 16:47         ` Nadav Amit
2018-10-18 17:00           ` Andy Lutomirski
2018-10-18 17:25             ` Nadav Amit
2018-10-18 17:29               ` Andy Lutomirski
2018-10-18 17:42                 ` Nadav Amit
2018-10-19  1:08             ` Nadav Amit
2018-10-19  4:29               ` Andy Lutomirski
2018-10-19  4:44                 ` Nadav Amit
2018-10-20  1:22                   ` Masami Hiramatsu
2018-10-19  5:00                 ` Alexei Starovoitov
2018-10-19  8:22                   ` Peter Zijlstra [this message]
2018-10-19 14:47                     ` Alexei Starovoitov
2018-10-19  8:19                 ` Peter Zijlstra
2018-10-19 10:38                 ` Oleg Nesterov
2018-10-19  8:33               ` Peter Zijlstra
2018-10-19 14:29                 ` Andy Lutomirski
2018-11-29  9:46                   ` Peter Zijlstra
2018-10-18  7:54     ` Peter Zijlstra
2018-10-18 18:14       ` Nadav Amit
2018-10-18  0:54 ` [RFC PATCH 2/5] x86: patch indirect branch promotion Nadav Amit
2018-10-18  0:54 ` [RFC PATCH 3/5] x86: interface for accessing indirect branch locations Nadav Amit
2018-10-18  0:54 ` [RFC PATCH 4/5] x86: learning and patching indirect branch targets Nadav Amit
2018-10-18  0:54 ` [RFC PATCH 5/5] x86: relpoline: disabling interface Nadav Amit
2018-10-23 18:36 ` [RFC PATCH 0/5] x86: dynamic indirect call promotion Dave Hansen
2018-10-23 20:32   ` Nadav Amit
2018-10-23 20:37     ` Dave Hansen
2018-11-28 16:08 ` Josh Poimboeuf
2018-11-28 19:34   ` Nadav Amit
2018-11-29  0:38     ` Josh Poimboeuf
2018-11-29  1:40       ` Andy Lutomirski
2018-11-29  2:06         ` Nadav Amit
2018-11-29  3:24           ` Andy Lutomirski
2018-11-29  4:36             ` Josh Poimboeuf
2018-11-29  6:06             ` Andy Lutomirski
2018-11-29 15:19               ` Josh Poimboeuf
2018-12-01  6:52                 ` Nadav Amit
2018-12-01 14:25                   ` Josh Poimboeuf

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