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Subject: Re: [PATCH 4/5] rcuwait: Introduce rcuwait_active()
To:     Davidlohr Bueso <dave@stgolabs.net>, tglx@linutronix.de
Cc:     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>
References: <20200424054837.5138-1-dave@stgolabs.net>
 <20200424054837.5138-5-dave@stgolabs.net>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <57309494-58bf-a11e-e4ac-e669e6af22f2@redhat.com>
Date:   Mon, 18 May 2020 12:33:00 +0200
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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>

diff --git a/include/linux/rcuwait.h b/include/linux/rcuwait.h
index c1414ce44abc..61c56cca95c4 100644
--- a/include/linux/rcuwait.h
+++ b/include/linux/rcuwait.h
@@ -31,7 +31,7 @@ static inline void rcuwait_init(struct rcuwait *w)
  */
 static inline int rcuwait_active(struct rcuwait *w)
 {
-	return !!rcu_dereference(w->task);
+	return !!rcu_access_pointer(w->task);
 }
 
 extern int rcuwait_wake_up(struct rcuwait *w);


Paolo