On Tue, Feb 18, 2014 at 06:23:47PM +0000, Peter Sewell wrote: > On 18 February 2014 17:16, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > On Tue, Feb 18, 2014 at 08:49:13AM -0800, Linus Torvalds wrote: > >> On Tue, Feb 18, 2014 at 7:31 AM, Torvald Riegel <triegel@xxxxxxxxxx> wrote: > >> > On Mon, 2014-02-17 at 16:05 -0800, Linus Torvalds wrote: > >> >> And exactly because I know enough, I would *really* like atomics to be > >> >> well-defined, and have very clear - and *local* - rules about how they > >> >> can be combined and optimized. > >> > > >> > "Local"? > >> > >> Yes. > >> > >> So I think that one of the big advantages of atomics over volatile is > >> that they *can* be optimized, and as such I'm not at all against > >> trying to generate much better code than for volatile accesses. > >> > >> But at the same time, that can go too far. For example, one of the > >> things we'd want to use atomics for is page table accesses, where it > >> is very important that we don't generate multiple accesses to the > >> values, because parts of the values can be change *by*hardware* (ie > >> accessed and dirty bits). > >> > >> So imagine that you have some clever global optimizer that sees that > >> the program never ever actually sets the dirty bit at all in any > >> thread, and then uses that kind of non-local knowledge to make > >> optimization decisions. THAT WOULD BE BAD. > > > > Might as well list other reasons why value proofs via whole-program > > analysis are unreliable for the Linux kernel: > > > > 1. As Linus said, changes from hardware. > > > > 2. Assembly code that is not visible to the compiler. > > Inline asms will -normally- let the compiler know what > > memory they change, but some just use the "memory" tag. > > Worse yet, I suspect that most compilers don't look all > > that carefully at .S files. > > > > Any number of other programs contain assembly files. > > > > 3. Kernel modules that have not yet been written. Now, the > > compiler could refrain from trying to prove anything about > > an EXPORT_SYMBOL() or EXPORT_SYMBOL_GPL() variable, but there > > is currently no way to communicate this information to the > > compiler other than marking the variable "volatile". > > > > Other programs have similar issues, e.g., via dlopen(). > > > > 4. Some drivers allow user-mode code to mmap() some of their > > state. Any changes undertaken by the user-mode code would > > be invisible to the compiler. > > > > 5. JITed code produced based on BPF: https://lwn.net/Articles/437981/ > > > > And probably other stuff as well. > > interesting list. So are you saying that value-range-analysis and > such-like (I say glibly, without really knowing what "such-like" > refers to here) are fundamentally incompatible with > the kernel code, or can you think of some way to tell the compiler a > bound on the footprint of the "unseen" changes in each of those cases? Other than the "volatile" keyword, no. Well, I suppose you could also create a function that changed the variables in question, then arrange to never call it, but in such a way that the compiler could not prove that it was never called. But ouch! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
