From: Linus Torvalds <email@example.com> To: Hillf Danton <firstname.lastname@example.org> Cc: "Eric W. Biederman" <email@example.com>, Linux Kernel Mailing List <firstname.lastname@example.org>, Alexey Gladkov <email@example.com> Subject: Re: [GIT PULL] ucount fix for v5.14-rc Date: Sat, 7 Aug 2021 01:23:07 -0700 [thread overview] Message-ID: <CAHk-=whyqY=1cAOXTE1o=w2jm8CKcM47=iOR2o2aNundzUpa_g@mail.gmail.com> (raw) In-Reply-To: <firstname.lastname@example.org> On Fri, Aug 6, 2021 at 10:03 PM Hillf Danton <email@example.com> wrote: > > Then the current atomic_add_negative() in consideration over the "risk" > of count overflow in real workloads can be replaced with the not_zero > version. What? No. The atomic_add_negative() has absolutely nothing to do with not_zero. The "negative" comes not at all from the count ever being zero, and as I explained, that isn't even an issue here. The "negative" is from a large _positive_ count growing so much that the sign bit gets set. It's basically a "31-bit overflow" thing. So: - not_zero makes no sense for get_ucounts(), because it can't be zero, because we hold a reference to it - atomic_add_negative() is about not letting the counts become too large, and when they do, we undo the reference (ie the pattern is "increment ref - but if it then overflows into bit #31, decrement it again" and the two have *NOTHING* to do with each other. So your statement about replacing one with the other makes no sense. I was trying to explain that in _other_ situations, the "atomic_inc_not_zero()" kind of pattern is used as a way to allow the find-vs-last-drop race to be done without locking, but that's not what the ucounts code does. ucounts uses the ucounts_lock, and that one is entirely immaterial for the atomic_add_negative() case, because the "negative" test is literally about the value being as far away from zero as is _possible_ (and at that point, the lock is most definitely not needed - it's needed only for the cases where the refcount goes to zero, and to make sure that a "find" cannot race with that). Linus
next prev parent reply other threads:[~2021-08-07 8:23 UTC|newest] Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-08-05 17:15 Eric W. Biederman 2021-08-05 19:26 ` pr-tracker-bot [not found] ` <firstname.lastname@example.org> 2021-08-06 3:38 ` Eric W. Biederman [not found] ` <email@example.com> 2021-08-06 17:37 ` Linus Torvalds [not found] ` <firstname.lastname@example.org> 2021-08-07 8:23 ` Linus Torvalds [this message] [not found] ` <email@example.com> 2021-08-07 15:10 ` Linus Torvalds [not found] ` <firstname.lastname@example.org> 2021-08-08 1:00 ` Linus Torvalds [not found] ` <email@example.com> 2021-08-08 1:45 ` Linus Torvalds 2021-08-08 2:05 ` Linus Torvalds
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