On Tue, Feb 18, 2014 at 06:23:47PM +0000, Peter Sewell wrote:
> On 18 February 2014 17:16, Paul E. McKenney <paul...@linux.vnet.ibm.com>
> 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 <trie...@redhat.com> 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
