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Subject: Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()
From:   Alan Huang <mmpgouride@gmail.com>
In-Reply-To: <fedf0448966b44d5b9146508265874fd@AcuMS.aculab.com>
Date:   Fri, 21 Jul 2023 23:55:07 +0800
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "rcu@vger.kernel.org" <rcu@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        "roman.gushchin@linux.dev" <roman.gushchin@linux.dev>
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References: <04C1E631-725C-47AD-9914-25D5CE04DFF4@gmail.com>
 <fedf0448966b44d5b9146508265874fd@AcuMS.aculab.com>
To:     David Laight <David.Laight@ACULAB.COM>
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> 2023=E5=B9=B47=E6=9C=8821=E6=97=A5 19:51=EF=BC=8CDavid Laight =
<David.Laight@ACULAB.COM> =E5=86=99=E9=81=93=EF=BC=9A
>=20
> From: Alan Huang
>> Sent: 20 July 2023 19:54
>>=20
>> I noticed a commit c87a124a5d5e(=E2=80=9Cnet: force a reload of first =
item in hlist_nulls_for_each_entry_rcu=E2=80=9D)
>> and a related discussion [1].
>=20
> Hmmm... that was all about the retry loop in ipv4/udp.c
>=20
> AFAICT that retry got deleted by ca065d0c.
>=20
> That also changes the list from hlist_nulls_xxx to hlist_xxx.
> (I'm not sure of the difference)
>=20
> This might be why we're seeing unexpected 'port unreachable' messages?
>=20
> Quite why that has just started happening is another issue.
> Most of the UDP sockets we create aren't 'connected' so I don't
> believe they get moved between hash chains - just deleted.
> The deletion should leave the hash chain intact.

Thanks for the information! :)

>=20
> David
>=20
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, =
MK1 1PT, UK
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