linux-kernel - Re: [PATCH] cgroup: pids: use {READ,WRITE}

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Message-ID: <20191016142756.GN18794@devbig004.ftw2.facebook.com>
Date:   Wed, 16 Oct 2019 07:27:56 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Aleksa Sarai <cyphar@...har.com>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] cgroup: pids: use {READ,WRITE}_ONCE for pids->limit
 operations

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?  The alternative would be seqlock or
u64_stats or straight-up locking but idk for this atomic64_t should be
fine.

> > The non-RMW ops are (typically) regular LOADs and STOREs and are
> > canonically implemented using READ_ONCE(), WRITE_ONCE(),
> > smp_load_acquire() and smp_store_release() respectively. 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.
> 
> 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.

Thanks.

-- 
tejun