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:
> > xagsmtp2.20140303190831.9...@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?
