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:
> > xagsmtp2.20140303204700.3...@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:
> > > > xagsmtp2.20140303190831.9...@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?
>
> 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
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