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    SubjectRe: [RFC][PATCH 0/5] arch: atomic rework
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    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?



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