On Fri, Jun 03, 2022 at 04:12:37PM +0200, Paul Heidekrüger wrote: > 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 :-) You're right; strictly speaking this isn't a control dependency. In fact it's not a dependency at all, just an ordering restriction that's connected with a conditional test. If you would like to submit a patch updating the text, please feel free to do so. Alan
