netdev - RE: [PATCH v2 net] ipv4: update fib_info

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Date:   Mon, 17 Jan 2022 09:20:16 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
CC:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        syzbot <syzkaller@...glegroups.com>,
        Ido Schimmel <idosch@...lanox.com>,
        "Jiri Pirko" <jiri@...lanox.com>
Subject: RE: [PATCH v2 net] ipv4: update fib_info_cnt under spinlock
 protection

From: Eric Dumazet
> Sent: 16 January 2022 09:02
> 
> From: Eric Dumazet <edumazet@...gle.com>
> 
> In the past, free_fib_info() was supposed to be called
> under RTNL protection.
> 
> This eventually was no longer the case.
> 
> Instead of enforcing RTNL it seems we simply can
> move fib_info_cnt changes to occur when fib_info_lock
> is held.

This probably ought to be a stable candidate.

If an increment is lost due to the unlocked inc/dec it is
possible for the count to wrap to -1 if all fib are deleted.
That will cause the table size to be doubled on every
allocate (until the count goes +ve again).

Losing a decrement is less of a problem.
You'd need to lose a lot of them for any ill effect.

	David

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