Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 8 Nov 2019 09:01:31 -0800 | Subject | Re: KCSAN: data-race in __alloc_file / __alloc_file |
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On Fri, Nov 8, 2019 at 5:28 AM Eric Dumazet <eric.dumazet@gmail.com> 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.
Linus
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