linux-kernel - Re: [PATCH 17/17] RCU'd vfsmounts

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Message-ID: <20131004061503.GC5790@linux.vnet.ibm.com>
Date:	Thu, 3 Oct 2013 23:15:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 17/17] RCU'd vfsmounts

On Thu, Oct 03, 2013 at 11:03:05PM -0700, Josh Triplett wrote:
> On Thu, Oct 03, 2013 at 10:29:59PM -0700, Paul E. McKenney wrote:
> > On Thu, Oct 03, 2013 at 04:28:27PM -0700, Josh Triplett wrote:
> > > On Thu, Oct 03, 2013 at 01:52:45PM -0700, Linus Torvalds wrote:
> > > > On Thu, Oct 3, 2013 at 1:41 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > > > >
> > > > > The problem is this:
> > > > > A = 1, B = 1
> > > > > CPU1:
> > > > > A = 0
> > > > > <full barrier>
> > > > > synchronize_rcu()
> > > > > read B
> > > > >
> > > > > CPU2:
> > > > > rcu_read_lock()
> > > > > B = 0
> > > > > read A
> > 
> > /me scratches his head...
> > 
> > OK, for CPU2 to see 1 from its read from A, the corresponding RCU
> > read-side critical section must have started before CPU1 did A=0.  This
> > means that this same RCU read-side critical section must have started
> > before CPU1's synchronize_rcu(), which means that it must complete
> > before that synchronize_rcu() returns.  Therefore, CPU2's B=0 must
> > execute before CPU1's read of B, hence that read of B must return zero.
> > 
> > Conversely, if CPU1's read from B returns 1, we know that CPU2's
> > RCU read-side critical section must not have completed until after
> > CPU1's synchronize_rcu() returned, which means that the RCU read-side
> > critical section must have started after that synchronize_rcu() started,
> > so CPU1's assignment to A must also have already happened.  Therefore,
> > CPU2's read from A must return zero.
> 
> Yeah, that makes sense.
> 
> I think too much time spent staring at the *implementation* of RCU and
> the exciting assumptions it has to make about barriers or memory
> operations leaking out of the implementations of the RCU primitives (for
> instance, the fun needed to guarantee a memory barrier on all CPUs, or
> to safely use non-atomic operations inside RCU itself) makes it entirely
> too difficult to look at a perfectly ordinary *use* of RCU primitives
> and see the obvious. :)

I must confess that my first thought upon seeing Al's example was "but
of course CPU2's write to B and read from A can be reordered by either
the compiler or the CPU!"  I had to look again myself.  ;-)

							Thanx, Paul

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