netdev - Re: [PATCH] sock: Make sock->sk

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Date:   Sat, 22 Dec 2018 08:34:55 -0800
From:   Deepa Dinamani <deepa.kernel@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Linux Network Devel Mailing List <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        y2038 Mailman List <y2038@...ts.linaro.org>
Subject: Re: [PATCH] sock: Make sock->sk_tstamp thread-safe

> Are we actually worried about concurrent writers here? I thought the
> only problem was a race between writer and reader, which would mean
> that we could solve it using only a seqcount_t which is cheaper to
> update than a seqlock_t.

I considered using just the seqcount_t. But, I think we do care about
concurrent writers here.
A couple of scenarios I can think of:

1. When you have 2 concurrent recvmsg() calls on a socket, and they
both try to update sk_tstamp.
2. If a socket has don't have one of the SO_TIMESTAMP/NS options set
and you have a first recvmsg and a concurrent ioctl call on the
socket.

These are corner cases and if we don't care aout these then we can use
just the sequence counters.

I have missed out tstamp update in the sunrcpc code. If everyone is ok
with this approach, I will add it in when I post an update

-Deepa