netdev - Re: [PATCH] inet: frags: Remove unnecessary smp_store_release/READ

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Date:   Fri, 31 May 2019 08:45:47 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH] inet: frags: Remove unnecessary
 smp_store_release/READ_ONCE



On 5/31/19 7:45 AM, Herbert Xu wrote:
> On Fri, May 31, 2019 at 10:24:08AM +0200, Dmitry Vyukov wrote:
>>
>> OK, let's call it barrier. But we need more than a barrier here then.
> 
> READ_ONCE/WRITE_ONCE is not some magical dust that you sprinkle
> around in your code to make it work without locks.  You need to
> understand exactly why you need them and why the code would be
> buggy if you don't use them.
> 
> In this case the code doesn't need them because an implicit
> barrier() (which is *stronger* than READ_ONCE/WRITE_ONCE) already
> exists in both places.
>

More over, adding READ_ONCE() while not really needed prevents some compiler
optimizations.

( Not in this particular case, since fqdir->dead is read exactly once, but we could
have had a loop )

I have already explained that the READ_ONCE() was a leftover of the first version
of the patch, that I refined later, adding correct (and slightly more complex) RCU
barriers and rules.

Dmitry, the self-documentation argument is perfectly good, but Herbert
put much nicer ad hoc comments.