Re: [PATCH RFC] locking: Add volatile to arch_spinlock_t structures

From: Paul E. McKenney
Date: Thu Dec 04 2014 - 13:36:44 EST

On Thu, Dec 04, 2014 at 10:02:14AM -0800, Linus Torvalds wrote:
> On Wed, Dec 3, 2014 at 11:02 PM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, Dec 03, 2014 at 10:40:45PM -0800, Linus Torvalds wrote:
> >> On Dec 3, 2014 10:31 PM, "Linus Torvalds" <torvalds@xxxxxxxxxxxxxxxxxxxx>
> >> wrote:
> >> >
> >> > So no, no, no. C got this wrong. Volatile data structures are a
> >> fundamental mistake and a bug.
> >>
> >> BTW, I'm not at all interested in language lawyering and people who say
> >> "but but we can do x". A compiler that modifies adjacent fields because the
> >> standard leaves is open is a crap compiler, and we won't use it, or disable
> >> the broken optimization. It is wrong from a concurrency standpoint anyway,
> >> and adding broken volatiles is just making things worse.
> >
> > Understood, for example, adjacent fields protected by different locks
> > as one example, where adjacent-field overwriting completely breaks even
> > very conservatively designed code.
> Exactly. Compilers that "optimize" things to touch fields that aren't
> touched by the source code are simply inherently buggy shit. I'm not
> at all interested in catering to their insanity.
> It doesn't matter one whit if they can point to the legacy C "virtual
> machine" definition and say that those accesses are invisible in the
> virtual machine. They are not invisible in real life, and it is
> entirely possible that two adjacent variables or fields are protected
> by different locks - even in non-kernel code. Claiming that they need
> to be marked volatile is a symptom of a diseased compiler writer.
> Now, the one exception to this is generally bitfields, because there
> the programmer knowingly and intentionally puts the fields together in
> the same storage unit. I also think that volatile bitfields are an
> insane concept, even if I think that the standard allows them. So I am
> not saying that compilers should try to magically make bitfield
> members not access the members around them.
> I also accept that some architectures are broken. Old
> non-byte/word-access alpha being the really canonical example. It's
> not the compilers fault if the architecture is broken, and the
> compiler cannot magically fix it.

I have to ask... Does this mean we can remove the current
restrictions against 8-bit and 16-bit access from smp_load_acquire()
and smp_store_release()?

> But compilers that think that "hey, vectorization is cool, and I can
> do load-stores and mask things dynamically" are misguided crap. It may
> be fancy, it may be really cool compiler technology, but it's
> fundamentally wrong unless the programmer told it was safe some way
> (be it with a "pragma" or "restrict" or a compile-time switch or
> whatever).

You might be happy to hear that I just sent an email to the C++ standards
committee noting that valid C11 compilers are not supposed to be able to
introduce data races [1]. I further argued that any store-widening access
affecting any non-private adjacent variable must be assumed to (illegally
per the C11 standard) introduce a data race, even if there are no locks,
atomic accesses, transactions, or any other synchronization mechanism
anywhere in that translation unit. After all, any non-static function
in that translation unit might be called from some other translation
unit that -did- use locking or whatever.

I will let you know how it goes. ;-)

Thanx, Paul

[1] A "data race" occurs in any C11 program where multiple threads
might be accessing a non-atomic variable, and where at least
one of the accesses is a write. C11 states that data races
result in undefined behavior. Therefore, if the source code
does not contain a data race, the object code had also better
be free of data races. Otherwise, the compiler inflicted
undefined behavior on a perfectly legitimate program.

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