[ltt-dev] [PATCH] Fix dirty page accounting in redirty_page_for_writepage()

[Christoph Lameter]

On Thu, 30 Apr 2009, Ingo Molnar wrote:

> > may not care too much about a event counter missing a beat once in
> > a while for platforms not supporting atomic per cpu ops. I know
> > this affects IA64. The cost of an atomic operations for an event
> > counter update (which would have avoided the potential of a
> > concurrent update) was not justifiable.
>
> when you say "atomics", do you mean the classic meaning of atomics?
> Because there are no classic atomics involved. This is the
> before/after disassembly from Eric's commit 4e69489a0:

The fallback for IA64 would be to use full (classic) atomic operations
(fetchadd) instead of fast atomic vs. interrupt as available on x86

> c0436275:   64 83 05 20 5f 6a c0    addl   $0x1,%fs:0xc06a5f20
>
> There's no atomic instructions at all - the counters here are only
> accessed locally. They are local-irq-atomic, but not
> cacheline-atomic.

Right but that is not available on IA64. So one must choose between
manually disabling interrupts and then increment the counter (long code
sequence) and a classic atomic operation for the fallback.