Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier()

[Paul E. McKenney]

On Wed, Jun 17, 2015 at 05:11:09PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 17, 2015 at 07:57:12AM -0700, Paul E. McKenney wrote:
> > On Wed, Jun 17, 2015 at 02:29:24PM +0200, Peter Zijlstra wrote:
> > > I did leave off the READ/WRITE ONCE stuff, because I could not come up
> > > with a scenario where it makes a difference -- I appreciate paranoia,
> > > but I also think we should not overdo the thing.
> > 
> > I can only conclude that you have not read this document:
> > 
> > 	http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4455.html
> 
> This would be correct.
> 
> > Specifically, please keep in mind that unless you mark either the variable
> > or the memory access, the compiler is within its rights to assume that
> > there are no concurrent accesses to that variable.  For but one example,
> > if you do a normal store to a given variable, then the compiler is
> > within its rights to use that variable as temporary storage prior to
> > that store.  And yes, you can reasonably argue that no sane compiler
> > would store something else to s->sequence given that it could free up
> > a register by storing the incremented value, but the fact remains that
> > you have given it permission to do so if it wants.
> 
> Argh *grmbl*, that's bloody insane!

You expected me to argue with that statement?  ;-)

> So I get the re-loading, I get the tearing, but this random intermittent
> values (somewhat related to stores out of thin air) is completely
> bonkers.
> 
> I would very much prefer a compiler switch that instructs the compiler
> to not do bloody stupid things like this instead of marking every other
> load/store in the kernel with volatile.

I would of course be good with such a compiler switch, though my earlier
attempts to negotiate one were unsuccessful.  But I don't believe that we
discussed a switch to specifically prohibit only use of to-be-stored-into
variables as temporary scratch space.  The trick is finding restrictions
that are useful, but that don't imply -O0.

Any GCC or LLVM folks on the list?

> Note that if GCC were to actually do something like this, the kernel
> would already be broken, because I'm very sure we did not consider/audit
> it for this.

An accident waiting to happen, given that both GCC and the Linux kernel
are moving targets.  :-/

							Thanx, Paul

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