Re: [RFC][PATCH 0/5] arch: atomic rework

[Peter Sewell]

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?

Peter


>                                                         Thanx, Paul
>
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