On Sat, May 25, 2019 at 12:43:40AM +0200, Andrea Parri wrote:
Subject: Documentation/atomic_t.txt: Clarify pure non-rmw usage
Clarify that pure non-RMW usage of atomic_t is pointless, there is nothing 'magical' about atomic_set() / atomic_read().
This is something that seems to confuse people, because I happen upon it semi-regularly.
Acked-by: Will Deacon will.deacon@arm.com Reviewed-by: Greg Kroah-Hartman gregkh@linuxfoundation.org Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org
Documentation/atomic_t.txt | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt index dca3fb0554db..89eae7f6b360 100644 --- a/Documentation/atomic_t.txt +++ b/Documentation/atomic_t.txt @@ -81,9 +81,11 @@ SEMANTICS The non-RMW ops are (typically) regular LOADs and STOREs and are canonically implemented using READ_ONCE(), WRITE_ONCE(), smp_load_acquire() and -smp_store_release() respectively. +smp_store_release() respectively. Therefore, if you find yourself only using +the Non-RMW operations of atomic_t, you do not in fact need atomic_t at all +and are doing it wrong.
The counterargument (not so theoretic, just look around in the kernel!) is: we all 'forget' to use READ_ONCE() and WRITE_ONCE(), it should be difficult or more difficult to forget to use atomic_read() and atomic_set()... IAC, I wouldn't call any of them 'wrong'.
I'm thinking you mean that the type system isn't helping us with READ/WRITE_ONCE() like it does with atomic_t ? And while I agree that there is room for improvement there, that doesn't mean we should start using atomic*_t all over the place for that.
Part of the problem with READ/WRITE_ONCE() is that it serves a dual purpose; we've tried to untangle that at some point, but Linus wasn't having it.