Re: page_waitqueue() considered harmful

[Nicholas Piggin <npiggin@gmail.com>]

On Thu, 29 Sep 2016 08:21:32 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Sep 29, 2016 at 11:31:32AM +1000, Nicholas Piggin wrote:
> > > Since the {set,clear}_bit operations are atomic, they must be ordered
> > > against one another. The subsequent test_bit is a load, which, since its
> > > to the same variable, and a CPU must appear to preserve Program-Order,
> > > must come after the RmW.
> > > 
> > > So I think you're right and that we can forgo the memory barriers here.
> > > I even think this must be true on all architectures.  
> > 
> > In generic code, I don't think so. We'd need an
> > smp_mb__between_bitops_to_the_same_aligned_long, wouldn't we?
> > 
> > x86 implements set_bit as 'orb (addr),bit_nr', and compiler could
> > implement test_bit as a byte load as well. If those bits are in
> > different bytes, then they could be reordered, no?
> > 
> > ia64 does 32-bit ops. If you make PG_waiter 64-bit only and put it
> > in the different side of the long, then this could be a problem too.  
> 
> Not on ia64, its atomics are full barriers too, just like x86 (even
> though its docs imply otherwise). But I get the point.

Oh yes of course I knew x86 atomics were barriers :)

Take Alpha instead. It's using 32-bit ops.

> I would however rather audit and attempt to fix affected archs before
> introducing such a barrier if at all possible.

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