From: Aleksa Sarai <cyphar@cyphar.com>
To: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>,
Johannes Weiner <hannes@cmpxchg.org>,
cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org
Subject: Re: [PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit operations
Date: Thu, 17 Oct 2019 02:35:20 +1100 [thread overview]
Message-ID: <20191016153520.zet5mn5xsygig4xc@yavin.dot.cyphar.com> (raw)
In-Reply-To: <20191016152946.34j5x45ko5auhv3g@yavin.dot.cyphar.com>
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On 2019-10-17, Aleksa Sarai <cyphar@cyphar.com> wrote:
> On 2019-10-16, Tejun Heo <tj@kernel.org> wrote:
> > Hello, Aleksa.
> >
> > On Wed, Oct 16, 2019 at 07:32:19PM +1100, Aleksa Sarai wrote:
> > > Maybe I'm misunderstanding Documentation/atomic_t.txt, but it looks to
> > > me like it's explicitly saying that I shouldn't use atomic64_t if I'm
> > > just using it for fetching and assignment.
> >
> > Hah, where is it saying that?
>
> Isn't that what this says:
>
> > Therefore, if you find yourself only using the Non-RMW operations of
> > atomic_t, you do not in fact need atomic_t at all and are doing it
> > wrong.
>
> Doesn't using just atomic64_read() and atomic64_set() fall under "only
> using the non-RMW operations of atomic_t"? But yes, I agree that any
> locking is overkill.
>
> > > As for 64-bit on 32-bit machines -- that is a separate issue, but from
> > > [1] it seems to me like there are more problems that *_ONCE() fixes than
> > > just split reads and writes.
> >
> > Your explanations are too wishy washy. If you wanna fix it, please do
> > it correctly. R/W ONCE isn't the right solution here.
>
> Sure, I will switch it to use atomic64_read() and atomic64_set() instead
> if that's what you'd prefer. Though I will mention that on quite a few
> architectures atomic64_read() is defined as:
>
> #define atomic64_read(v) READ_ONCE((v)->counter)
Though I guess that's because on those architectures it turns out that
READ_ONCE is properly atomic?
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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next prev parent reply other threads:[~2019-10-16 15:35 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-12 1:05 [PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit operations Aleksa Sarai
2019-10-14 15:41 ` Tejun Heo
2019-10-14 15:59 ` Aleksa Sarai
2019-10-14 16:33 ` Tejun Heo
2019-10-16 8:32 ` Aleksa Sarai
2019-10-16 14:27 ` Tejun Heo
2019-10-16 15:29 ` Aleksa Sarai
2019-10-16 15:32 ` Tejun Heo
2019-10-16 15:35 ` Aleksa Sarai [this message]
2019-10-16 15:54 ` Tejun Heo
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