On Wed, Feb 01, 2012 at 12:59:24PM -0800, Linus Torvalds wrote:
> On Wed, Feb 1, 2012 at 12:41 PM, Torvald Riegel <trie...@redhat.com> wrote:
> >
> > You do rely on the compiler to do common transformations I suppose:
> > hoist loads out of loops, CSE, etc. How do you expect the compiler to
> > know whether they are allowed for a particular piece of code or not?
>
> We have barriers.
>
> Compiler barriers are cheap. They look like this:
>
> include/linux/compiler-gcc.h:
> #define barrier() __asm__ __volatile__("": : :"memory")
>
> and if we don't allow hoisting, we'll often use something like that.
>
> It's not the only thing we do. We have cases where it's not that you
> can't hoist things outside of loops, it's that you have to read things
> exactly *once*, and then use that particular value (ie the compiler
> can't be allowed to reload the value because it may change). So any
> *particular* value we read is valid, but we can't allow the compiler
> to do anything but a single access.
>
> So we have this beauty:
>
> include/linux/compiler.h:
> #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
My (perhaps forlorn and naive) hope is that C++11 memory_order_relaxed
will eventually allow ACCESS_ONCE() to be upgraded so that (for example)
access-once increments can generate a single increment-memory instruction
on x86.
> which does that for us (quite often, it's not used as-is: a more
> common case is using the "rcu_access_pointer()" inline function that
> not only does that ACCESS_ONCE() but also has the capability to enable
> run-time dynamic verification that you are in a proper RCU read locked
> section etc)
And I also hope that rcu_access_pointer(), rcu_dereference(), and
friends can eventually be written in terms of C++11 memory_order_consume
so that the some of the less-sane optimizations can actually be applied
without breaking the kernel.
Of course, this cannot happen immediately. First, gcc must fully support
the C++11 features of interest, the inevitable bugs must be found and
fixed, and the optimizer will no doubt need to be reworked a bit before
the kernel can make use of these features.
New architectures might eventually might define things like atomic_inc()
in terms of C++11 atomics, but let's start with the straightforward stuff
as and if it makes sense.
Thanx, Paul
> We also have tons of (very subtle) code that actually creates locks
> and memory ordering etc. They usually just use the big "barrier()"
> approach to telling the compiler not to combine or move memory
> accesses around it.
>
> And I realize that compiler people tend to think that loop hoisting
> etc is absolutely critical for performance, and some big hammer like
> "barrier()" makes a compiler person wince. You think it results in
> horrible code generation problems.
>
> It really doesn't. Loops are fairly unusual in the kernel to begin
> with, and the compiler barriers are a total non-issue. We have much
> more problems with the actual CPU barriers that can be *very*
> expensive on some architectures, and we work a lot at avoiding those
> and avoiding cacheline ping-pong issues etc.
>
> >> > No vague assumptions with lots of hand-waving.
> >>
> >> So here's basically what the kernel needs:
> >>
> >> - if we don't touch a field, the compiler doesn't touch it.
> >
> > "we don't touch a field": what does that mean precisely? Never touch it
> > in the same thread? Same function? Same basic block? Between two
> > sequence points? When crossing synchronizing code? For example, in the
> > above code, can we touch it earlier/later?
>
> Why are you trying to make it more complex than it is?
>
> If the source code doesn't write to it, the assembly the compiler
> generates doesn't write to it.
>
> Don't try to make it anything more complicated. This has *nothing* to
> do with threads or functions or anything else.
>
> If you do massive inlining, and you don't see any barriers or
> conditionals or other reasons not to write to it, just write to it.
>
> Don't try to appear smart and make this into something it isn't.
>
> Look at the damn five-line example of the bug. FIX THE BUG. Don't try
> to make it anything bigger than a stupid compiler bug. Don't try to
> make this into a problem it simply isn't.
>
> Linus
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majord...@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/
>
