----- On Aug 17, 2019, at 4:44 AM, Linus Torvalds torva...@linux-foundation.org wrote:
> > But I'm seeing a lot of WRITE_ONCE(x, constantvalue) kind of things > and don't seem to find a lot of reason to think that they are any > inherently better than "x = constantvalue". If the only states that "x" can take is 1 or 0, then indeed there seems to be no point in using a WRITE_ONCE() when paired with a READ_ONCE() other than for documentation purposes. However, if the state of "x" can be any pointer value, or a reference count value, then not using "WRITE_ONCE()" to store a constant leaves the compiler free to perform that store in more than one memory access. Based on [1], section "Store tearing", there are situations where this happens on x86 in the wild today when storing 64-bit constants: the compiler is then free to decide to use two 32-bit immediate store instructions. Thanks, Mathieu [1] https://lwn.net/Articles/793253/ -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
