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.) > 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