linux-kernel - Re: [v3,11/41] mips: reuse asm-generic/barrier.h

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Message-ID: <20160126194410.GQ4503@linux.vnet.ibm.com>
Date:	Tue, 26 Jan 2016 11:44:10 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>,
	Will Deacon <will.deacon@....com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	virtualization@...ts.linux-foundation.org,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Joe Perches <joe@...ches.com>,
	David Miller <davem@...emloft.net>, linux-ia64@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-metag@...r.kernel.org, linux-mips@...ux-mips.org,
	x86@...nel.org, user-mode-linux-devel@...ts.sourceforge.net,
	adi-buildroot-devel@...ts.sourceforge.net,
	linux-sh@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
	xen-devel@...ts.xenproject.org, Ralf Baechle <ralf@...ux-mips.org>,
	Ingo Molnar <mingo@...nel.org>, ddaney.cavm@...il.com,
	james.hogan@...tec.com, Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [v3,11/41] mips: reuse asm-generic/barrier.h

On Tue, Jan 26, 2016 at 11:24:02AM +0100, Peter Zijlstra wrote:
> On Thu, Jan 14, 2016 at 02:20:46PM -0800, Paul E. McKenney wrote:
> > On Thu, Jan 14, 2016 at 01:24:34PM -0800, Leonid Yegoshin wrote:
> > > On 01/14/2016 12:48 PM, Paul E. McKenney wrote:
> > > >
> > > >So SYNC_RMB is intended to implement smp_rmb(), correct?
> > > Yes.
> > > >
> > > >You could use SYNC_ACQUIRE() to implement read_barrier_depends() and
> > > >smp_read_barrier_depends(), but SYNC_RMB probably does not suffice.
> > > 
> > > If smp_read_barrier_depends() is used to separate not only two reads
> > > but read pointer and WRITE basing on that pointer (example below) -
> > > yes. I just doesn't see any example of this in famous
> > > Documentation/memory-barriers.txt and had no chance to know what you
> > > use it in this way too.
> > 
> > Well, Documentation/memory-barriers.txt was intended as a guide for Linux
> > kernel hackers, and not for hardware architects.
> 
> Yeah, this goes under the header: memory-barriers.txt is _NOT_ a
> specification (I seem to keep repeating this).
> 
> > ------------------------------------------------------------------------
> > 
> > commit 955720966e216b00613fcf60188d507c103f0e80
> > Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Date:   Thu Jan 14 14:17:04 2016 -0800
> > 
> >     documentation: Subsequent writes ordered by rcu_dereference()
> >     
> >     The current memory-barriers.txt does not address the possibility of
> >     a write to a dereferenced pointer.  This should be rare, 
> 
> How are these rare? Isn't:
> 
> 	rcu_read_lock()
> 	obj = rcu_dereference(ptr);
> 	if (!atomic_inc_not_zero(&obj->ref))
> 		obj = NULL;
> 	rcu_read_unlock();
> 
> a _very_ common thing to do?

It is, but it provides its own barriers, so does not need to rely on
dependency ordering.

							Thanx, Paul