Re: [PATCH] tools/memory-model: document the "one-time init" pattern
From: Eric Biggers <ebiggers@kernel.org>
Date: 2020-07-18 01:02:53
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On Fri, Jul 17, 2020 at 01:51:38PM -0400, Alan Stern wrote:
On Fri, Jul 17, 2020 at 06:47:50PM +0100, Matthew Wilcox wrote:quoted
On Thu, Jul 16, 2020 at 09:44:27PM -0700, Eric Biggers wrote:...quoted
quoted
+ /* on success, pairs with smp_load_acquire() above and below */ + if (cmpxchg_release(&foo, NULL, p) != NULL) {Why do we have cmpxchg_release() anyway? Under what circumstances is cmpxchg() useful _without_ having release semantics?To answer just the last question: cmpxchg() is useful for lock acquisition, in which case it needs to have acquire semantics rather than release semantics.
To clarify, there are 4 versions of cmpxchg: cmpxchg(): does ACQUIRE and RELEASE (on success) cmpxchg_acquire(): does ACQUIRE only (on success) cmpxchg_release(): does RELEASE only (on success) cmpxchg_relaxed(): no barriers The problem here is that here we need RELEASE on success and ACQUIRE on failure. But no version guarantees any barrier on failure. So as far as I can tell, the best we can do is use cmpxchg_release() (or cmpxchg() which would be stronger but unnecessary), followed by a separate ACQUIRE on failure. - Eric