Re: [PATCH 07/11] io_uring: use atomic_t for refcounts

From: Kees Cook <keescook@chromium.org>
To: Will Deacon <will@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>, Jann Horn <jannh@google.com>,
	io-uring <io-uring@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH 07/11] io_uring: use atomic_t for refcounts
Date: Wed, 11 Dec 2019 08:56:44 -0800	[thread overview]
Message-ID: <201912110851.88536F3F@keescook> (raw)
In-Reply-To: <20191211102012.GA4123@willie-the-truck>

On Wed, Dec 11, 2019 at 10:20:13AM +0000, Will Deacon wrote:
> On Tue, Dec 10, 2019 at 03:55:05PM -0700, Jens Axboe wrote:
> > On 12/10/19 3:46 PM, Kees Cook wrote:
> > > On Tue, Dec 10, 2019 at 03:21:04PM -0700, Jens Axboe wrote:
> > >> On 12/10/19 3:04 PM, Jann Horn wrote:
> > >>> [context preserved for additional CCs]
> > >>>
> > >>> On Tue, Dec 10, 2019 at 4:57 PM Jens Axboe <axboe@kernel.dk> wrote:
> > >>>> Recently had a regression that turned out to be because
> > >>>> CONFIG_REFCOUNT_FULL was set.
> > >>>
> > >>> I assume "regression" here refers to a performance regression? Do you
> > >>> have more concrete numbers on this? Is one of the refcounting calls
> > >>> particularly problematic compared to the others?
> > >>
> > >> Yes, a performance regression. io_uring is using io-wq now, which does
> > >> an extra get/put on the work item to make it safe against async cancel.
> > >> That get/put translates into a refcount_inc and refcount_dec per work
> > >> item, and meant that we went from 0.5% refcount CPU in the test case to
> > >> 1.5%. That's a pretty substantial increase.
> > >>
> > >>> I really don't like it when raw atomic_t is used for refcounting
> > >>> purposes - not only because that gets rid of the overflow checks, but
> > >>> also because it is less clear semantically.
> > >>
> > >> Not a huge fan either, but... It's hard to give up 1% of extra CPU. You
> > >> could argue I could just turn off REFCOUNT_FULL, and I could. Maybe
> > >> that's what I should do. But I'd prefer to just drop the refcount on the
> > >> io_uring side and keep it on for other potential useful cases.
> > > 
> > > There is no CONFIG_REFCOUNT_FULL any more. Will Deacon's version came
> > > out as nearly identical to the x86 asm version. Can you share the
> > > workload where you saw this? We really don't want to regression refcount
> > > protections, especially in the face of new APIs.
> > > 
> > > Will, do you have a moment to dig into this?
> > 
> > Ah, hopefully it'll work out ok, then. The patch came from testing the
> > full backport on 5.2.

Oh good! I thought we had some kind of impossible workload. :)

> > Do you have a link to the "nearly identical"? I can backport that
> > patch and try on 5.2.
> 
> You could try my refcount/full branch, which is what ended up getting merged
> during the merge window:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/will/linux.git/log/?h=refcount/full

Yeah, as you can see in the measured tight-loop timings in
https://git.kernel.org/linus/dcb786493f3e48da3272b710028d42ec608cfda1
there was 0.1% difference for Will's series compared to the x86 assembly
version, where as the old FULL was almost 70%.

-- 
Kees Cook