Messages in this thread | | | Date | Tue, 27 Feb 2018 13:06:35 +0800 | From | Boqun Feng <> | Subject | Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and spin_unlock() |
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On Mon, Feb 26, 2018 at 04:24:27PM +0000, Will Deacon wrote: > 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)); >
Hmm.. this is new to me. So the write part of spin_lock() and the WRITE_ONCE() will not get reordered? Could you explain more about this or point where I should look in the document? I understand the write part of spin_lock() must be committed earlier than the WRITE_ONCE() committed due to the ll/sc, but I thought the ordering of their arrivals in memory system is undefined/arbitary.
Regards, Boqun
> and that could fire. Is that relied on somewhere? > > Will [unhandled content-type:application/pgp-signature] | |