On Fri, Nov 8, 2019 at 9:01 AM Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Fri, Nov 8, 2019 at 5:28 AM Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote: > > > > Linus, what do you think of the following fix ? > > I think it's incredibly ugly. > > I realize that avoiding the cacheline dirtying might be worth it, but > I'd like to see some indication that it actually matters and helps > from a performance angle. We've already dirtied memory fairly close, > even if it might not share a cacheline (that structure is randomized, > we've touched - or will touch - 'cred->usage') too. > > Honestly, I don't think get_cred() is even in a hotpath. Most cred use > just use the current cred that doesn't need the 'get'. So the > optimization looks somewhat questionable - for all we know it just > makes things worse. > > I also don't like using a "WRITE_ONCE()" without a reason for it. In > this case, the only "reason" is that KCSAN special-cases that thing. > I'd much rather have some other way to mark it. > > So it just looks hacky to me. > > I like that people are looking at KCSAN, but I get a very strong > feeling that right now the workarounds for KCSAN false-positives are > incredibly ugly, and not always appropriate. > > There is absolutely zero need for a WRITE_ONCE() in this case. The > code would work fine if the compiler did the zero write fifty times, > and re-ordered it wildly. We have a flag that starts out set, and we > clear it. There's really no "write-once" about it. > Ok, so what do you suggest next ? Declare KCSAN useless because too many false positives ?
