Messages in this thread | | | Date | Mon, 26 Feb 2018 16:24:27 +0000 | From | Will Deacon <> | Subject | Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and spin_unlock() |
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On Mon, Feb 26, 2018 at 08:06:59AM -0800, Linus Torvalds wrote: > 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.
Strictly speaking, that's not what we've got implemented on arm64: only the read part of the RmW has Acquire semantics, but there is a total order on the lock/unlock operations for the lock. For example, if one CPU does:
spin_lock(&lock); WRITE_ONCE(foo, 42);
then another CPU could do:
if (smp_load_acquire(&foo) == 42) BUG_ON(!spin_is_locked(&lock));
and that could fire. Is that relied on somewhere?
Will
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