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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 25 Apr 2021 09:39:49 -0700
Message-ID: <CAHk-=wi2NEfJmgZbqHL43a2rsWuc7Oyb5kfiz9x8z+S-juL7-w@mail.gmail.com>
Subject: Re: [GIT PULL] locking/urgent for v5.12
To:     Borislav Petkov <bp@suse.de>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ali Saidi <alisaidi@amazon.com>,
        Steve Capper <steve.capper@arm.com>,
        Will Deacon <will@kernel.org>, Waiman Long <longman@redhat.com>
Cc:     x86-ml <x86@kernel.org>, lkml <linux-kernel@vger.kernel.org>
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Oh, and replying to myself only because I spazzed out and pressed
"send" before I had filled out the full participants line.

Sorry for the duplicate message quoted in full below.

                 Linus

On Sun, Apr 25, 2021 at 9:37 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> [ Side note: this is cc'd to x86-ml, even though x86 is the _one_
> architecture that was guaranteed to be not at all affected by the
> actual locking bug, since a locked op is always ordered on x86. ]
>
> On Sun, Apr 25, 2021 at 2:39 AM Borislav Petkov <bp@suse.de> wrote:
> >
> >   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git tags/locking_urgent_for_v5.12
> >
> > - Fix ordering in the queued writer lock's slowpath.
>
> So I'm looking at that change, because the code is confusing.
>
> Why did it add that "cnts" variable? We know it must have the value
> _QW_WAITING, since that's what the atomic_cond_read_relaxed() waits
> for.
>
> I'm assuming it's because of the switch to try_cmpxchg by PeterZ?
>
> HOWEVER.
>
> That actually just makes the code even MORE unreadable.
>
> That code was odd and hard to read even before, but now it's
> positively confusing.
>
> New confusion:
>  - Why is the truly non-critical cmpxchg using "try_cmpxhg()", when
>    the _first_ cmpxchg - above the loop - is not?
>
> Pre-existing confusion:
>  - Why is the code using "atomic_add()" to set a bit?
>
> Yeah, yeah, neither of these are *bugs*, but Christ is that code hard
> to read. The "use add to set a bit" is valid because of the spinlock
> serialization (ie only one add can ever happen), and the
> cmpxchg-vs-try_cmpxchg confusion isn't buggy, it's just really really
> confusing that that same function is using two different - but
> equivalent - cmpxchg things on the same variable literally a couple of
> lines apart.
>
> I've pulled this, but can we please
>
>  - make *both* of the cmpxchg's use "try_cmpxchg()" (and thus that
>    "cnts" variable)?
>
>  - add a comment about _why_ it's doing "atomic_add()" instead of the
>    much more logical "atomic_or()", and about how the spinlock serializes
>    it
>
> I'm assuming the "atomic_add()" is simply because many more
> architectures have that as an actual intrinsic atomic. I understand.
> But it's really really not obvious from the code.
>
>                 Linus