RE: [RFC][PATCH 3/3] locking/qspinlock: Optimize for x86

[David Laight]

From: Peter Zijlstra
> Sent: 26 September 2018 12:01
> 
> On x86 we cannot do fetch_or with a single instruction and end up
> using a cmpxchg loop, this reduces determinism. Replace the fetch_or
> with a very tricky composite xchg8 + load.
> 
> The basic idea is that we use xchg8 to test-and-set the pending bit
> (when it is a byte) and then a load to fetch the whole word. Using
> two instructions of course opens a window we previously did not have.
...

IIRC the load will be 'slow' because it will have to wait for the
earlier store to actually complete - rather than being satisfied
by data from the store buffer (because the widths are different).

This may not matter for xchg ?

	David

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