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

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Date:   Mon, 18 May 2020 12:33:00 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Davidlohr Bueso <dave@...olabs.net>, tglx@...utronix.de
Cc:     peterz@...radead.org, maz@...nel.org, bigeasy@...utronix.de,
        rostedt@...dmis.org, torvalds@...ux-foundation.org,
        will@...nel.org, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 4/5] rcuwait: Introduce rcuwait_active()

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>

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