On Wed 07-11-18 09:44:00, Vlastimil Babka wrote: > On 11/7/18 9:20 AM, Michal Hocko wrote: > > On Tue 06-11-18 21:51:47, Arun KS wrote: > > Hi, > > there's typo in subject: evaluvations -> evaluations. > > However, "fix" is also misleading (more below), so I'd suggest something > like: > > mm: reference totalram_pages and managed_pages once per function > > >> This patch is in preparation to a later patch which converts totalram_pages > >> and zone->managed_pages to atomic variables. This patch does not introduce > >> any functional changes. > > > > I forgot to comment on this one. The patch makes a lot of sense. But I > > would be little bit more conservative and won't claim "no functional > > changes". As things stand now multiple reads in the same function are > > racy (without holding the lock). I do not see any example of an > > obviously harmful case but claiming the above is too strong of a > > statement. I would simply go with something like "Please note that > > re-reading the value might lead to a different value and as such it > > could lead to unexpected behavior. There are no known bugs as a result > > of the current code but it is better to prevent from them in principle." > > However, the new code doesn't use READ_ONCE(), so the compiler is free > to read the value multiple times, and before the patch it was free to > read it just once, as the variables are not volatile. So strictly > speaking this is indeed not a functional change (if compiler decides > differently based on the patch, it's an implementation detail). Yes, compiler is allowed to optimize this either way without READ_ONCE but it is allowed to do two reads so claiming no functional change is a bit problematic. Not that this would be a reason to discuss this in length... -- Michal Hocko SUSE Labs
