linux-kernel - Re: [PATCH 4/5] rcuwait: Introduce rcuwait

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Date:   Tue, 19 May 2020 08:34:24 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Davidlohr Bueso <dave@...olabs.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Marc Zyngier <maz@...nel.org>,
        Sebastian Sewior <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        will@...nel.org, joel@...lfernandes.org,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 4/5] rcuwait: Introduce rcuwait_active()

On Mon, 18 May 2020 at 18:36, Paolo Bonzini <pbonzini@...hat.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@...hat.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@...il.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

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