On Tue, May 28, 2019 at 12:47:19PM +0200, Peter Zijlstra wrote: > 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@xxxxxxx> > > > Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > > > --- > > > 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 ? Yep. > 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. Agreed. But this still doesn't explain that "and are doing it wrong", AFAICT; maybe just remove that part? Andrea > > 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.
