Hi all, I was going through litmus-tests.txt and came across the following: > LIMITATIONS > =========== > > Limitations of the Linux-kernel memory model (LKMM) include: > > 1.Compiler optimizations are not accurately modeled. Of course, > the use of READ_ONCE() and WRITE_ONCE() limits the compiler's > ability to optimize, but under some circumstances it is possible > for the compiler to undermine the memory model. For more > information, see Documentation/explanation.txt (in particular, > the "THE PROGRAM ORDER RELATION: po AND po-loc" and "A WARNING" > sections). > > Note that this limitation in turn limits LKMM's ability to > accurately model address, control, and data dependencies. > For example, if the compiler can deduce the value of some variable > carrying a dependency, then the compiler can break that dependency > by substituting a constant of that value. > > Conversely, LKMM sometimes doesn't recognize that a particular > optimization is not allowed, and as a result, thinks that a > dependency is not present (because the optimization would break it). > The memory model misses some pretty obvious control dependencies > because of this limitation. A simple example is: > > r1 = READ_ONCE(x); > if (r1 == 0) > smp_mb(); > WRITE_ONCE(y, 1); > > There is a control dependency from the READ_ONCE to the WRITE_ONCE, > even when r1 is nonzero, but LKMM doesn't realize this and thinks > that the write may execute before the read if r1 != 0. (Yes, that > doesn't make sense if you think about it, but the memory model's > intelligence is limited.) I'm unclear as to why the documentation sees a control dependency from the READ_ONCE() to the WRITE_ONCE() here. Quoting from explanation.txt: > Finally, a read event and another memory access event are linked by a > control dependency if the value obtained by the read affects whether > the second event is executed at all. Architectures might consider this control-dependent, yes, but since the value of the if condition does not affect whether the WRITE_ONCE() is executed at all, I'm not sure why this should be considered control-dependent in LKMM? I might have another question about explanation.txt's definition of control dependencies as per above, but will address it more thoroughly in another email :-) Many thanks, Paul
