Re: [patch V2 2/7] futex: Hash private futexes per process

From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Darren Hart <darren@dvhart.com>, Ingo Molnar <mingo@kernel.org>,
	Michael Kerrisk <mtk.manpages@googlemail.com>,
	Davidlohr Bueso <dave@stgolabs.net>, Chris Mason <clm@fb.com>,
	"Carlos O'Donell" <carlos@redhat.com>,
	Torvald Riegel <triegel@redhat.com>,
	Eric Dumazet <edumazet@google.com>
Subject: Re: [patch V2 2/7] futex: Hash private futexes per process
Date: Mon, 30 May 2016 10:43:46 +0200	[thread overview]
Message-ID: <20160530084346.GM3192@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20160527165211.GB28561@breakpoint.cc>

On Fri, May 27, 2016 at 06:52:11PM +0200, Sebastian Andrzej Siewior wrote:
> On 2016-05-19 14:21:48 [+0200], Peter Zijlstra wrote:
> > > +static void futex_populate_hash(unsigned int hash_bits)
> > > +{
> …
> > > +	raw_spin_lock(&mm->futex_hash.lock);
> > > +	/* We might have raced with another task allocating the hash. */
> > > +	if (!mm->futex_hash.hash) {
> > > +		mm->futex_hash.hash_bits = hash_bits;
> > > +		/*
> > > +		 * Ensure that the above is visible before we store
> > > +		 * the pointer.
> > > +		 */
> > > +		smp_wmb(); /* (A0) Pairs with (B) */
> > > +		mm->futex_hash.hash = hb;
> > 
> > 		smp_store_release(&mm->futex_hash.hash, hb); ?
> 
> just to be clear: You suggest to use "smp_store_release()" instead
> smp_wmb() followed by the assignment?

Yes, smp_store_release() is the most natural way to publish things like
this. Note that rcu_assign_pointer() also switched to using that. See
commit: 88c1863066cc ("rcu: Define rcu_assign_pointer() in terms of
smp_store_release()") for detail on the difference.