Messages in this thread | | | From | Linus Torvalds <> | Date | Mon, 26 Feb 2018 08:06:59 -0800 | Subject | Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and spin_unlock() |
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On Mon, Feb 26, 2018 at 6:21 AM, Luc Maranget <luc.maranget@inria.fr> wrote: > > That is, locks are not implemented from more basic primitive but are specified. > The specification can be described as behaving that way: > - A lock behaves as a read-modify-write. the read behaving as a read-acquire
This is wrong, or perhaps just misleading.
The *whole* r-m-w acts as an acquire. Not just the read part. The write is very much part of it.
Maybe that's what you meant, but it read to me as "just the read part of the rmw behaves as a read-acquire".
Because it is very important that the _write_ part of the rmw is also ordered wrt everything that is inside the spinlock.
So doing a spinlock as
(a) read-locked-acquire modify (c) write-conditional
would be wrong, because the accesses inside the spinlock are ordered not just wrt the read-acquire, they have to be ordered wrt the write too.
So it is closer to say that it's the _write_ of the r-m-w sequence that has the acquire semantics, not the read.
> - A unlock behaves as a store release.
Yes.
Linus
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