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Date:   Fri, 29 Nov 2019 08:04:10 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Marco Elver <elver@google.com>
Cc:     peterz@infradead.org, mingo@redhat.com, will@kernel.org,
        linux-kernel@vger.kernel.org, Qian Cai <cai@lca.pw>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] spinlock_debug: Fix various data races
Message-ID: <20191129070410.GA23979@gmail.com>
References: <20191120155715.28089-1-elver@google.com>
 <20191121183257.GA124760@gmail.com>
 <20191121194418.GA239776@google.com>
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* Marco Elver <elver@google.com> wrote:

> On Thu, 21 Nov 2019, Ingo Molnar wrote:
> 
> > 
> > * Marco Elver <elver@google.com> wrote:
> > 
> > >  static inline void debug_spin_lock_after(raw_spinlock_t *lock)
> > >  {
> > > -	lock->owner_cpu = raw_smp_processor_id();
> > > -	lock->owner = current;
> > > +	WRITE_ONCE(lock->owner_cpu, raw_smp_processor_id());
> > > +	WRITE_ONCE(lock->owner, current);
> > >  }
> > 
> > debug_spin_lock_after() runs inside the spinlock itself - why do these 
> > writes have to be WRITE_ONCE()?
> > 
> > > @@ -197,8 +197,8 @@ static inline void debug_write_unlock(rwlock_t *lock)
> > >  	RWLOCK_BUG_ON(lock->owner != current, lock, "wrong owner");
> > >  	RWLOCK_BUG_ON(lock->owner_cpu != raw_smp_processor_id(),
> > >  							lock, "wrong CPU");
> > > -	lock->owner = SPINLOCK_OWNER_INIT;
> > > -	lock->owner_cpu = -1;
> > > +	WRITE_ONCE(lock->owner, SPINLOCK_OWNER_INIT);
> > > +	WRITE_ONCE(lock->owner_cpu, -1);
> > >  }
> > 
> > This too is running inside the critical section of the spinlock - why are 
> > the WRITE_ONCE() calls necessary?
> 
> Although the writes are inside the critical section, they are read
> concurrently outside the critical section, e.g. in
> debug_spin_lock_before(). In other words, the WRITE_ONCE pair with the
> READ_ONCE that are *outside* the critical section.

Fair enough!

Thanks,

	Ingo