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

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

On Mon, Mar 03, 2014 at 07:55:08PM +0100, Torvald Riegel wrote:
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> 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.)

> I don't think the wording is flawed.  We could raise the requirement of
> having more than one value left for r1 to having more than N with N > 1
> values left, but the fundamental problem remains in that a compiler
> could try to generate a (big) switch statement.
> 
> Instead, I think that this indicates that the value_dep_preserving type
> modifier would be useful: It would tell the compiler that it shouldn't
> transform this into a branch in this case, yet allow that optimization
> for all other code.

Understood!

BTW, my current task is generating examples using the value_dep_preserving
type for RCU-protected array indexes.

							Thanx, Paul