Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 18 May 2020 09:37:00 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     tglx@linutronix.de, peterz@infradead.org, maz@kernel.org,
        bigeasy@linutronix.de, rostedt@goodmis.org,
        torvalds@linux-foundation.org, will@kernel.org,
        joel@joelfernandes.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH 4/5] rcuwait: Introduce rcuwait_active()
Message-ID: <20200518163700.4nn4csjlbpcixmsv@linux-p48b>
References: <20200424054837.5138-1-dave@stgolabs.net>
 <20200424054837.5138-5-dave@stgolabs.net>
 <57309494-58bf-a11e-e4ac-e669e6af22f2@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Disposition: inline
In-Reply-To: <57309494-58bf-a11e-e4ac-e669e6af22f2@redhat.com>
User-Agent: NeoMutt/20180716
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, 18 May 2020, Paolo Bonzini 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>

Acked-by: Davidlohr Bueso <dbueso@suse.de>