From: Luck, Tony > Sent: 13 June 2022 17:27 > > >> It's listed in Documentation/atomic_bitops.txt. > > > > Oh, so my memory was actually correct that I saw it in the docs > > somewhere. > > WDYT, should I mention this here in the code (block comment) as well > > that it's atomic and must not lose `volatile` as Andy suggested or > > it's sufficient to have it in the docs (+ it's not underscored)? > > I think a comment that the "volatile" is required to prevent re-ordering > is enough. > > But maybe others are sufficiently clear on the meaning? I once wasted > time looking for the non-atomic __test_bit() version (to use in some code > that was already protected by a spin lock, so didn't need the overhead > of an "atomic" version) before realizing there wasn't a non-atomic one. Does it make any sense for 'test bit' to be atomic? I'm not even sure is needs any ordering constraints either. The result is always stale - the value can be changed by another cpu at any time. The set/clear atomic bit-ops require a RMW bus cycle - which has to be locked (or similar) to avoid corruption. The atomic 'test and set' (etc) are RMW and return a valid state. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
