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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Tue, 19 May 2020 08:34:24 +0800
Message-ID: <CANRm+CxXacjtTZe8gGLR+iH_X8QrC2R_2ncL9aMB2ukoEmn8FQ@mail.gmail.com>
Subject: Re: [PATCH 4/5] rcuwait: Introduce rcuwait_active()
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Davidlohr Bueso <dave@stgolabs.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Marc Zyngier <maz@kernel.org>,
        Sebastian Sewior <bigeasy@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        will@kernel.org, joel@joelfernandes.org,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Davidlohr Bueso <dbueso@suse.de>
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On Mon, 18 May 2020 at 18:36, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 24/04/20 07:48, Davidlohr Bueso wrote:
> > +/*
> > + * Note: this provides no serialization and, just as with waitqueues,
> > + * requires care to estimate as to whether or not the wait is active.
> > + */
> > +static inline int rcuwait_active(struct rcuwait *w)
> > +{
> > +     return !!rcu_dereference(w->task);
> > +}
>
> This needs to be changed to rcu_access_pointer:
>
>
> --------------- 8< -----------------
> From: Paolo Bonzini <pbonzini@redhat.com>
> Subject: [PATCH] rcuwait: avoid lockdep splats from rcuwait_active()
>
> rcuwait_active only returns whether w->task is not NULL.  This is
> exactly one of the usecases that are mentioned in the documentation
> for rcu_access_pointer() where it is correct to bypass lockdep checks.
>
> This avoids a splat from kvm_vcpu_on_spin().
>
> Reported-by: Wanpeng Li <kernellwp@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Tested-by: Wanpeng Li <wanpengli@tencent.com>