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SubjectRe: andrea buffer code (2.2.9-C.gz)
Ingo Molnar wrote:
> > Yes but I am not worried, and being more finegrined in SMP is more fun :-).
> you might not be worried but others are ... Manfred has asked good
> questions: is the particular lock you have 'expanded' into per-object
> locks contended in tests, have you measured things. OS development is
> never 'mathematically clean'.

I've checked the code, it seems that contention is impossible:
The lock is only acquired in 'end_io_async()', and that
function is usually called from 'end_that_request_first'
which must be called with the io_request_lock acquired.

Ok, we don't want to rely on that, the interface should be SMP safe,
but one global spinlock is sufficient.

Andrea wrote:
> Since the wastage will be only in SMP I don't care too much, I
> would prefer to remove the buffer field that is useless instead
> of dropping a nice per-pagemap spinlock.

That's an invalid argument: remove both if that's possible, but
you cannot argue 'because I saved memory with the buffer/inode field
I can waste the memory somewhere else'.


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