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

[Torvald Riegel <triegel@redhat.com>]

On Wed, 2014-03-05 at 10:15 -0800, Paul E. McKenney wrote:
> On Wed, Mar 05, 2014 at 05:54:59PM +0100, Torvald Riegel wrote:
> > On Tue, 2014-03-04 at 13:35 -0800, Paul E. McKenney wrote:
> > > 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?
> > 
> > That's a good question, and one that as far as I understand currently,
> > essentially boils down to whether we want to have tight restrictions on
> > which operations are still vdp.
> > 
> > If we look at the different combinations, then it seems we can't decide
> > on whether we have a value-dependency just due to a vdp type:
> > * non-vdp || vdp:  vdp iff non-vdp == false
> > * vdp || non-vdp:  vdp iff non-vdp == false?
> > * vdp || vdp: always vdp? (and dependency on both?)
> > 
> > I'm not sure it makes sense to try to not make all of those
> > vdp-by-default.  The first and second case show that it's dependent on
> > the specific execution anyway, and thus is already covered by the
> > requirement that the value must still matter.   The vdp type is just a
> > way to prevent inappropriate compiler optimizations; it's not critical
> > for correctness is we make more stuff vdp, yet it may prevent some
> > optimizations in the affected expression.
> > 
> > If the compiler knows that some vdp-typed evaluation will not have a
> > value-dependency anyway, then it can just optimize this evaluation like
> > non-vdp code.
> > 
> > I guess not much would change for the code you posted, because we
> > already have to evaluate || operands in order, I believe (e.g., don't
> > access rdev->flags before doing the rdev == NULL check, modulo as-if).
> > Do I understand your question correctly?
> 
> Let me give an example for the other side:
> 
> 	struct foo value_dep_preserving *p;
> 	struct foo value_dep_preserving *q;
> 
> 	p = rcu_dereference(gp);
> 	q = rcu_dereference(gq);
> 	return myarray[p || q]];  /* Linux kernel doesn't do this. */
> 
> If we wanted this to work (and I am not at all convinced that we do),
> the compiler would have to force a data dependency through the "||".

Yes.

> But I would be just as happy to instead just say that boolean logical
> operators ("||" and "&&") never return vdp values.

I think those aren't actually the problem (or if they were, we'd need to
think about & and | on 1-bit integers or bitfields as well), but ...

> Ditto for the
> relational operators ("==", "!=", ">", ">=", "<", and "<=").  No one
> seems to rely on value dependencies via these operators, after all,
> and preserving value dependencies through them seems to require that
> the compiler generate odd code.

... that any conversion from vdp to bool requires specialized handling
by the compiler.  That happens on implicit conversion (as in "p || q")
and in the operators you mentioned.

I don't see a reason why conversion to bool (or any other operator
returning bool and taking vdp as operand) should be *always* non-vdp.
But it seems it would be easier to misuse than other operators.