Re: [RFC][PATCH 0/5] arch: atomic rework

["Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>]

On Tue, Mar 04, 2014 at 11:00:32AM -0800, Paul E. McKenney wrote:
> On Mon, Mar 03, 2014 at 09:46:19PM +0100, Torvald Riegel wrote:
> > xagsmtp2.20140303204700.3556@vmsdvma.vnet.ibm.com
> > X-Xagent-Gateway: vmsdvma.vnet.ibm.com (XAGSMTP2 at VMSDVMA)
> > 
> > On Mon, 2014-03-03 at 11:20 -0800, Paul E. McKenney wrote:
> > > On Mon, Mar 03, 2014 at 07:55:08PM +0100, Torvald Riegel wrote:
> > > > xagsmtp2.20140303190831.9500@uk1vsc.vnet.ibm.com
> > > > X-Xagent-Gateway: uk1vsc.vnet.ibm.com (XAGSMTP2 at UK1VSC)
> > > > 
> > > > On Fri, 2014-02-28 at 16:50 -0800, Paul E. McKenney wrote:
> > > > > +o	Do not use the results from the boolean "&&" and "||" when
> > > > > +	dereferencing.	For example, the following (rather improbable)
> > > > > +	code is buggy:
> > > > > +
> > > > > +		int a[2];
> > > > > +		int index;
> > > > > +		int force_zero_index = 1;
> > > > > +
> > > > > +		...
> > > > > +
> > > > > +		r1 = rcu_dereference(i1)
> > > > > +		r2 = a[r1 && force_zero_index];  /* BUGGY!!! */
> > > > > +
> > > > > +	The reason this is buggy is that "&&" and "||" are often compiled
> > > > > +	using branches.  While weak-memory machines such as ARM or PowerPC
> > > > > +	do order stores after such branches, they can speculate loads,
> > > > > +	which can result in misordering bugs.
> > > > > +
> > > > > +o	Do not use the results from relational operators ("==", "!=",
> > > > > +	">", ">=", "<", or "<=") when dereferencing.  For example,
> > > > > +	the following (quite strange) code is buggy:
> > > > > +
> > > > > +		int a[2];
> > > > > +		int index;
> > > > > +		int flip_index = 0;
> > > > > +
> > > > > +		...
> > > > > +
> > > > > +		r1 = rcu_dereference(i1)
> > > > > +		r2 = a[r1 != flip_index];  /* BUGGY!!! */
> > > > > +
> > > > > +	As before, the reason this is buggy is that relational operators
> > > > > +	are often compiled using branches.  And as before, although
> > > > > +	weak-memory machines such as ARM or PowerPC do order stores
> > > > > +	after such branches, but can speculate loads, which can again
> > > > > +	result in misordering bugs.
> > > > 
> > > > Those two would be allowed by the wording I have recently proposed,
> > > > AFAICS.  r1 != flip_index would result in two possible values (unless
> > > > there are further constraints due to the type of r1 and the values that
> > > > flip_index can have).
> > > 
> > > And I am OK with the value_dep_preserving type providing more/better
> > > guarantees than we get by default from current compilers.
> > > 
> > > One question, though.  Suppose that the code did not want a value
> > > dependency to be tracked through a comparison operator.  What does
> > > the developer do in that case?  (The reason I ask is that I have
> > > not yet found a use case in the Linux kernel that expects a value
> > > dependency to be tracked through a comparison.)
> > 
> > Hmm.  I suppose use an explicit cast to non-vdp before or after the
> > comparison?
> 
> That should work well assuming that things like "if", "while", and "?:"
> conditions are happy to take a vdp.  This assumes that p->a only returns
> vdp if field "a" is declared vdp, otherwise we have vdps running wild
> through the program.  ;-)
> 
> The other thing that can happen is that a vdp can get handed off to
> another synchronization mechanism, for example, to reference counting:
> 
> 	p = atomic_load_explicit(&gp, memory_order_consume);
> 	if (do_something_with(p->a)) {
> 		/* fast path protected by RCU. */
> 		return 0;
> 	}
> 	if (atomic_inc_not_zero(&p->refcnt) {
> 		/* slow path protected by reference counting. */
> 		return do_something_else_with((struct foo *)p);  /* CHANGE */
> 	}
> 	/* Needed slow path, but raced with deletion. */
> 	return -EAGAIN;
> 
> I am guessing that the cast ends the vdp.  Is that the case?

And here is a more elaborate example from the Linux kernel:

	struct md_rdev value_dep_preserving *rdev;  /* CHANGE */

	rdev = rcu_dereference(conf->mirrors[disk].rdev);
	if (r1_bio->bios[disk] == IO_BLOCKED
	    || rdev == NULL
	    || test_bit(Unmerged, &rdev->flags)
	    || test_bit(Faulty, &rdev->flags))
		continue;

The fact that the "rdev == NULL" returns vdp does not force the "||"
operators to be evaluated arithmetically because the entire function
is an "if" condition, correct?

							Thanx, Paul