linux-kernel - Re: WARNING: suspicious RCU usage in idtentry

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Message-ID: <87tuzzm8du.fsf@nanos.tec.linutronix.de>
Date:   Fri, 29 May 2020 10:51:25 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dmitry Vyukov <dvyukov@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     syzbot <syzbot+3ae5eaae0809ee311e75@...kaller.appspotmail.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: WARNING: suspicious RCU usage in idtentry_exit

Dmitry,

Dmitry Vyukov <dvyukov@...gle.com> writes:
> On Thu, May 28, 2020 at 10:48 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>> On Thu, May 28, 2020 at 10:19:02PM +0200, Thomas Gleixner wrote:
>> OK, from the .config, another suggestion is to build the kernel
>> with CONFIG_RCU_EQS_DEBUG=y.  This still requires that this issue be
>> reproduced, but it might catch the problem earlier.
>
> How much does it slow down execution? If we enable it on syzbot, it
> will affect all fuzzing done by syzbot always.
> It can tolerate significant slowdown and it's far from a production
> kernel (it enables KASAN, KCOV, LOCKDEP and more). But I am still
> asking because some debugging features are built without performance
> in mind at all (like let's just drop a global lock in every
> kmalloc/free, which may be too much even for a standard debug build).

It's not worse than lockdep.

Thanks,

        tglx